Green Tea Extract (EGCG): The Complete Supplement Guide

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Overview

The Basics

Green tea extract is a concentrated form of the compounds found in green tea leaves, the same Camellia sinensis plant that has been consumed as a beverage for thousands of years across East Asia. What makes green tea extract different from simply drinking tea is the concentration: a single capsule can deliver the equivalent of several cups of green tea in terms of its active compounds, particularly a polyphenol called EGCG (epigallocatechin-3-gallate).

EGCG is the star player in green tea extract, making up roughly 50% to 80% of the catechin content. One brewed cup of green tea contains approximately 50 mg of EGCG, while typical supplement capsules deliver 200 to 500 mg per serving [1][2]. This concentration is the reason people turn to extract supplements rather than simply drinking more tea: achieving research-level doses through tea alone would require drinking 8 to 10 cups per day.

People take green tea extract for a wide range of reasons. The most common are weight management, antioxidant support, cardiovascular health, and general wellness. It has also generated significant research interest for its potential role in supporting metabolic health, cognitive function, and cellular protection [1][2][3]. However, the evidence quality varies considerably across these applications, and the supplement carries meaningful safety considerations, particularly at higher doses, that set it apart from simply drinking green tea.

One important distinction worth understanding upfront: green tea extract as a supplement behaves differently in your body than drinking brewed green tea. The concentrated, solid bolus form delivers catechins in a way that changes their absorption profile and, importantly, their safety profile. This distinction has prompted both the European Food Safety Authority (EFSA) and the U.S. Pharmacopeia (USP) to issue specific safety guidance for green tea extract supplements that does not apply to drinking tea in the traditional way [4][5].

The Science

Green tea extract is derived from the unfermented leaves of Camellia sinensis (family Theaceae), which undergo minimal oxidation during processing, preserving the polyphenolic catechin content. The major catechins include (-)-epigallocatechin-3-gallate (EGCG), (-)-epigallocatechin (EGC), (-)-epicatechin-3-gallate (ECG), and (-)-epicatechin (EC), with EGCG comprising 50-80% of the total catechin fraction [1][2].

EGCG (C22H18O11, MW 458.37 g/mol) is a flavonoid-3-ol polyphenol characterized by three aromatic rings interconnected by a pyran ring, with a trihydroxyl B-ring and a gallate ester moiety. This structural configuration confers potent electron-donating and free radical-scavenging capacity [2][3].

The catechin content of green tea varies substantially with cultivar, growing conditions, processing method, and extraction technique. Dried leaf catechin concentrations range from 1,600 to 20,320 mg per 100g (a 13-fold variation), while brewed infusion concentrations vary from 2.3 to 203 mg per 100g (an 88-fold variation) [4]. Commercial supplement formulations typically standardize to a specific EGCG percentage, most commonly 45-50% by weight, though actual catechin content can vary considerably between products. Testing of 97 dietary supplements found catechin concentrations ranging from 1 microgram to 486 mg per gram, with 40% of products not declaring catechin content on their labels [5].

Beyond catechins, green tea extract contains caffeine (typically 2-4% by weight unless decaffeinated), L-theanine, gallic acid, chlorogenic acid, and various flavonols including quercetin, kaempferol, and myricetin [1][2].

Chemical & Nutritional Identity

Major catechins in green tea extract (by relative abundance):

• EGCG (epigallocatechin-3-gallate): 50-80% of total catechins. The most biologically active and most extensively studied. Gallate ester enhances binding affinity to multiple protein targets.
• EGC (epigallocatechin): Second most abundant. Shares the trihydroxyl B-ring but lacks the gallate moiety.
• ECG (epicatechin-3-gallate): Contains gallate group but has a dihydroxyl rather than trihydroxyl B-ring.
• EC (epicatechin): Smallest and simplest catechin. Also found in dark chocolate and cocoa.

Additional bioactive components:

• Caffeine (2-4% by weight, unless decaffeinated)
• L-theanine (amino acid unique to tea)
• Gallic acid, chlorogenic acid
• Flavonols: quercetin, kaempferol, myricetin

Mechanism of Action

The Basics

Green tea extract works through EGCG, a molecule that interacts with your body in several ways at once. Rather than having one single target like many pharmaceuticals, EGCG is more like a Swiss Army knife, influencing multiple biological processes simultaneously.

The most well-known effect involves antioxidant protection. EGCG is a powerful scavenger of free radicals, the unstable molecules that can damage your cells over time. Its antioxidant capacity is considered more potent than that of vitamins C and E [2][3]. Think of free radicals as sparks from a fire. Your body produces them naturally during normal metabolism, but too many of them can cause damage to your DNA, proteins, and cell membranes. EGCG helps neutralize these sparks before they cause problems.

EGCG also influences how your body handles fat. It activates an enzyme called AMPK (a metabolic master switch) that stimulates fat burning and helps regulate blood sugar [3][6]. This is part of why green tea extract is associated with modest fat loss effects, particularly when combined with caffeine and exercise.

On the inflammation front, EGCG can dial down your body's inflammatory response by blocking a signaling pathway called NF-kB, which acts as a central switchboard for inflammation. When NF-kB is overly active, it drives chronic low-grade inflammation that contributes to many health problems. EGCG helps keep this pathway in check [2][3].

There is an important nuance to understand about EGCG's behavior at different concentrations. At the levels you would achieve from normal supplementation, it acts primarily as an antioxidant and protector. But at very high concentrations, it can actually flip and become a pro-oxidant, generating the same reactive oxygen species it normally neutralizes [2]. This is one of the reasons why "more is not better" with green tea extract.

The Science

EGCG exerts its biological effects through a remarkably diverse set of molecular targets, functioning as a pleiotropic bioactive compound with concentration-dependent activity profiles.

Antioxidant and pro-oxidant duality:
The antioxidant mechanism derives from the di- and tri-hydroxyl groups on the B-ring and the meta-5,7-dihydroxyl groups on the A-ring, which serve as electron donors and free radical traps [2][3]. EGCG scavenges superoxide, hydroxyl, peroxyl radicals, nitrogen oxide, lipid free radicals, singlet oxygen, and peroxynitrite. It also chelates transition metal ions (Cu2+, Zn2+, Fe2+) via the ortho-3',4'-dihydroxy moiety, preventing Fenton reaction-mediated oxidative damage [3]. At the enzymatic level, EGCG induces phase II detoxifying enzymes (GST, NQO, UDPGT) through Nrf2-ARE pathway activation while inhibiting phase I CYP1A1 and CYP2E expression [2]. Paradoxically, at supraphysiological concentrations, EGCG undergoes autoxidation to generate hydrogen peroxide and catecholquinone intermediates, producing pro-oxidant effects that contribute to its selective cytotoxicity against cancer cells [2][3].

NF-kB pathway inhibition:
EGCG blocks IKK phosphorylation, preventing IkBa degradation and p65 nuclear translocation, thereby suppressing transcription of pro-inflammatory and pro-survival genes [2][3]. This mechanism underlies its observed effects on COX-2 suppression, iNOS downregulation, and reduction of pro-inflammatory cytokines including TNF-a, IL-1, IL-6, and IL-8 [3][6].

AMPK activation and metabolic regulation:
EGCG activates the AMPK pathway, stimulating free fatty acid oxidation and lipolysis while suppressing hepatic gluconeogenesis [3][6]. It also inhibits pancreatic lipase activity, reducing dietary fat absorption, and suppresses the JAK2/STAT3 pathway involved in adipogenesis [6].

Epigenetic modulation:
EGCG functions as a direct inhibitor of DNMT1 by forming hydrogen bonds within the catalytic pocket, reversing hypermethylation of tumor suppressor genes (p16, RAR, MGMT, MLH1) [2]. It also modulates histone acetylation through effects on HAT and HDAC activities and regulates microRNA expression, upregulating tumor-suppressive miRNAs while inhibiting oncogenic variants [2].

Cell cycle and apoptosis regulation:
EGCG reduces cyclin D1, CDK2, CDK4, and CDK6 expression while increasing p16, p18, p21, p27, and p53 levels, promoting G0/G1 phase arrest. It activates the intrinsic apoptotic pathway through caspase-3, -8, and -9 activation and suppression of anti-apoptotic Bcl-2 and Bcl-XL [2]. Cancer cells demonstrate significantly higher sensitivity to these effects compared to normal cells [2].

Angiogenesis and metastasis inhibition:
EGCG reduces HIF-1a, VEGF, VEGFR1, and VEGFR2 expression while inhibiting MMP-2 and MMP-9 (IC50: 8-13 uM) and blocking urokinase plasminogen activator (uPA) through direct binding to catalytic residues [2].

Additional targets:

• Telomerase inhibition through galloyl radical-mediated covalent modification [2]
• Chymotrypsin-like proteasome inhibition (IC50: 0.09-0.2 uM cell-free) [2]
• Topoisomerase I inhibition (3-17 uM) without affecting topoisomerase II [2]
• Dihydrofolate reductase (DHFR) inhibition (Ki = 0.11 mM, tight binding in bovine liver) [2]
• Androgen receptor modulation via miRNA-21 and miRNA-330 regulation [2]

Absorption & Bioavailability

The Basics

Getting EGCG from a capsule into your bloodstream and tissues is not straightforward. Despite green tea extract being one of the most popular supplements in the world, EGCG has relatively poor oral bioavailability, meaning only a fraction of what you swallow actually reaches your cells in active form.

After you take a green tea extract capsule, EGCG reaches its peak levels in your blood within about one to two hours [3][6]. However, only about 20% of the ingested EGCG makes it into your bloodstream compared to what would arrive via direct injection [6]. The rest is broken down in your gut, metabolized by your liver, or excreted before it can be used.

Several factors influence how much EGCG your body absorbs:

• Food matters: Taking green tea extract with food reduces peak blood levels of EGCG. However, USP and EFSA recommend taking it with food because fasting, while increasing absorption, also increases the risk of liver stress [4][5]. This creates a practical tradeoff where slightly lower absorption is the safer choice.
• Caffeine synergy: The caffeine naturally present in most green tea extracts may enhance EGCG absorption and extend its activity window. Decaffeinated products may deliver somewhat different pharmacokinetic profiles.
• Milk proteins: Adding milk to green tea can bind to catechins and reduce their bioavailable fraction. This applies more to brewed tea than to capsules.
• Form matters: Not all green tea extract products deliver the same amount of usable EGCG. Standardized extracts with specified EGCG percentages offer more predictable dosing than unstandardized whole-leaf products.

EGCG has a relatively short half-life of approximately 1.9 to 4.6 hours [6], meaning most of it clears from your blood within half a day. This is why some research protocols use twice-daily dosing, though the safety implications of repeated dosing must be weighed carefully.

One notable property: EGCG can cross the blood-brain barrier, reaching neural tissue [6]. This is relevant to its potential neuroprotective effects, though the concentrations achieved in brain tissue are quite small relative to what is ingested.

The Science

Pharmacokinetics:
Following oral administration, EGCG reaches peak plasma concentrations (Tmax) at approximately 1.0 to 2.2 hours post-ingestion [6]. A 1600 mg dose produced a Cmax of 3392 ng/mL, though doses of 1g or higher are typically required to achieve plasma concentrations exceeding 1 uM, which is the threshold for many in vitro biological activities [6]. Oral bioavailability has been measured at approximately 20.3% relative to intravenous administration in humans, with significant interspecies variation (26.5% in CF-1 mice versus 1.6% in Sprague Dawley rats) [6].

Metabolism:
EGCG undergoes extensive first-pass metabolism through multiple pathways [6]:

• Methylation via catechol-O-methyltransferase (COMT), producing 4''-O-methyl-EGCG
• Glucuronidation by UDP-glucuronosyltransferases (UGT), with EGCG-4''-O-glucuronide as the primary metabolite
• Sulfation mediated by sulfotransferase (SULT) enzymes
• Ring fission biotransformation by colonic gut microbiota
• Enterohepatic recirculation enabling partial reabsorption

Excretion occurs primarily through bile, with only trace amounts appearing in urine. Tissue concentrations represent merely 0.0003 to 0.45% of the total ingested amount [6].

Distribution:
EGCG distributes to tissues including liver, lungs, kidneys, heart, and brain. It crosses the blood-brain barrier to reach neural tissue, where it chelates transition metal ions and scavenges reactive oxygen species [6]. However, tissue concentrations remain low relative to plasma levels.

Bioavailability enhancement strategies:
Emerging research on nanocarrier delivery systems, including lipid-based nanoparticles, liposomes, and chitosan-modified mucoadhesive ligands, has demonstrated significantly improved EGCG stability and bioavailability in preclinical models [6]. Piperine (BioPerine) co-administration may enhance absorption through inhibition of glucuronidation pathways, though clinical data in humans remains limited.

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Research & Clinical Evidence

The Basics

The research on green tea extract and EGCG spans thousands of studies, but there is a critical distinction between what has been shown in laboratory settings and what has been convincingly demonstrated in humans. Many of the most exciting findings come from cell cultures and animal studies, where researchers can use EGCG concentrations far higher than what the human body achieves through oral supplementation.

In the areas where human clinical trials exist, the evidence quality ranges from moderate to low across most health outcomes [1]. The strongest human evidence supports modest effects on cardiovascular risk markers (blood pressure, lipid profiles) and small contributions to fat oxidation when combined with caffeine. For cancer prevention, cognitive protection, and many other promoted benefits, the evidence remains preliminary or mixed.

Here is what the clinical picture looks like across the major research areas:

Cardiovascular health is where the most consistent human data exists. Clinical trials have shown that green tea extract can produce small but statistically significant reductions in blood pressure and improvements in cholesterol profiles, particularly in people who are overweight or have type 2 diabetes [6][7]. These effects include reduced LDL cholesterol, increased HDL cholesterol, and improved endothelial function.

Weight management has been extensively studied, with mixed results. Some trials show modest reductions in body weight, BMI, and waist circumference, particularly at doses below 500 mg EGCG over 12 weeks [6]. Others find no significant effect compared to placebo. The fat-burning effects appear to depend heavily on caffeine status: habitual caffeine consumers may see substantially less benefit [1].

Cancer prevention has generated enormous research interest. While preclinical data is compelling, showing EGCG's activity against multiple cancer types in cell and animal models, human clinical evidence remains limited. A few observational studies suggest associations between green tea consumption and reduced cancer risk, and small clinical trials have shown activity against cervical dysplasia and chronic lymphocytic leukemia, but large-scale prevention trials have produced inconsistent results [2][7].

The Science

Cardiovascular outcomes (Grade B evidence):
Meta-analyses of randomized controlled trials demonstrate that green tea catechin supplementation produces statistically significant reductions in systolic and diastolic blood pressure in prehypertensive and type 2 diabetic populations [6]. A three-month trial of green tea extract in obese hypertensive patients showed reductions in insulin resistance (HOMA-IR), inflammatory markers, and oxidative stress alongside blood pressure improvements [6]. Lipid profile improvements include decreased LDL cholesterol, total cholesterol, and triglycerides with increased HDL and adiponectin levels, with clinical significance achieved at doses exceeding 800 mg daily for 8 or more weeks [6].

Metabolic and weight management (Grade B-C evidence):
A randomized, double-blind, placebo-controlled trial demonstrated that EGCG supplementation (300 mg daily for 8 weeks) significantly improved fasting blood glucose, body weight, hs-CRP, and lipid profiles in type 2 diabetic patients, with elevated GLP-1 levels suggesting incretin pathway involvement [6]. Obesity studies show that daily EGCG consumption below 500 mg for 12 weeks significantly reduced body weight, BMI, and waist circumference [6].

Cancer prevention (Grade C-D evidence):
Preclinical data demonstrates EGCG's antitumor activity across breast, prostate, lung, colorectal, melanoma, and hematological cancers through apoptosis induction, cell cycle arrest, epigenetic modulation, angiogenesis suppression, and metastasis prevention [2][3]. EGCG shows cancer-selective cytotoxicity, with cancer cell lines demonstrating higher sensitivity than normal cell counterparts [2].

Human data includes a cervical dysplasia trial showing efficacy of 200 mg oral EGCG for 12 weeks in HPV-infected cervical lesions [2], and evidence that oral EGCG may induce biologic responses in chronic lymphocytic leukemia patients [7]. Breast cancer patients (stages I and II) in observational studies experienced lower recurrence rates with regular green tea consumption [2][7]. However, one prostate cancer prevention trial in high-risk men showed elevated incidence with supplementation, and some data suggest green tea consumption may elevate breast cancer risk in certain postmenopausal women [7].

Neuroprotective effects (Grade D evidence, primarily preclinical):
EGCG crosses the blood-brain barrier and demonstrates neuroprotective activity through ROS/RNS scavenging, transition metal chelation, prevention of amyloid plaque accumulation (Alzheimer's disease model), prevention of Lewy body accumulation (Parkinson's disease model), and suppression of microglial activation [6]. Human data on cognitive outcomes remains limited and inconsistent [1].

Antimicrobial activity (Grade D evidence, primarily in vitro):
EGCG demonstrates activity against MRSA, Mycobacterium tuberculosis, Helicobacter pylori, influenza viruses (A/H1N1, A/H3N2, B), HCV, HBV, HIV (reverse transcriptase inhibition), and Candida species [6]. Clinical translation of these findings remains limited.

Evidence & Effectiveness Matrix

Categories scored: 14
Categories with community data: 12
Categories not scored (insufficient data): Muscle Growth, Appetite & Satiety, Memory & Cognition, Anxiety, Stress Tolerance, Libido, Sexual Function, Joint Health, Pain Management, Recovery & Healing, Physical Performance, Gut Health, Digestive Comfort, Hair Health, Heart Rate & Palpitations, Hormonal Symptoms, Temperature Regulation, Fluid Retention, Bone Health, Cravings & Impulse Control

Benefits & Potential Effects

The Basics

Green tea extract offers a range of potential health benefits, though it is important to set realistic expectations. This is not a miracle supplement. The benefits tend to be modest, gradual, and most pronounced in people who have specific risk factors or deficiencies that green tea extract may help address.

The best-supported benefits relate to cardiovascular health. Regular green tea extract use has been associated with small improvements in blood pressure and cholesterol levels, particularly lower LDL (the "bad" cholesterol) and higher HDL (the "good" cholesterol) [6]. These effects are most meaningful for people who already have elevated cardiovascular risk factors.

For weight management, green tea extract may provide a small additional push when combined with a healthy diet and regular exercise. It is not effective as a standalone weight loss solution, and the effects are diminished in people who regularly consume caffeine [1][6]. The mechanism involves a modest increase in fat oxidation and energy expenditure, particularly during physical activity.

The antioxidant protection offered by EGCG is among its most consistently demonstrated effects. EGCG is a powerful neutralizer of free radicals, with antioxidant capacity exceeding that of vitamins C and E in laboratory comparisons [3][6]. While translating antioxidant activity from laboratory measurements to real-world health outcomes is complicated, the overall evidence supports a protective role against oxidative stress.

Emerging evidence also points toward potential benefits for blood sugar regulation in people with type 2 diabetes or prediabetes, improved endothelial function (the health of your blood vessel linings), and anti-inflammatory effects that may support general wellness [6].

The Science

Cardiovascular protection:
Green tea catechins reduce LDL cholesterol oxidation and inhibit intestinal cholesterol absorption [6]. Endothelial function improvement occurs through enhanced nitric oxide bioavailability and arterial dilation promotion. Myeloperoxidase inhibition reduces oxidative damage to the vascular wall. AMPK and mTOR pathway modulation affect vascular smooth muscle cell proliferation. Epigenetic effects include DNMT inhibition and HDAC1 expression reduction in endothelial cells [6].

Metabolic regulation:
EGCG protects pancreatic beta-cells from oxidative damage and apoptosis via miR-16-5p suppression [6]. It upregulates insulin receptor substrate-1 (IRS-1) phosphorylation, improving glucose tolerance and reducing postprandial glucose spikes. NLRP3 inflammasome targeting addresses high-fat diet-induced metabolic dysfunction, and selenoprotein P (SELENOP) suppression reduces hepatic insulin resistance [6].

Antioxidant and anti-inflammatory cascades:
Beyond direct radical scavenging, EGCG induces SOD, catalase, and glutathione peroxidase expression through Nrf2-ARE pathway activation [2][3]. COX-2 and iNOS downregulation occurs through NF-kB and AP-1 blockade. JAK1/2 signaling pathway inhibition reduces pro-inflammatory gene expression [6]. In ulcerative colitis models, EGCG reduced disease severity through TLR4/MyD88/NF-kB pathway targeting [6].

Anti-senescence activity:
EGCG mitigates oxidative stress-induced senescence in human mesenchymal stem cells through p53-p21 pathway downregulation and Nrf2 upregulation [6]. Cellular senescence delay has been observed in vascular smooth muscle cells, dermal fibroblasts, and articular chondrocytes at 50-100 uM concentrations through PI3K/Akt/mTOR and AMPK pathway regulation [6].

Anticancer mechanisms:
EGCG exhibits antitumor effects across multiple cancer types through apoptosis induction (caspase-3 activation, cytochrome c release, PARP cleavage), cell cycle inhibition, epigenetic modulation (DNMT inhibition reactivating p16, RAR, MGMT, hMLH1), angiogenesis suppression (HIF-1a and NF-kB inhibition), metastasis prevention (MMP-2/9 inhibition, uPA blockade), and telomerase repression [2][3][6]. Synergistic potential with conventional chemotherapy agents has been demonstrated in preclinical models [6].

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Side Effects & Safety

The Basics

Green tea extract has an important safety profile that separates it from many other supplements. At typical doses (200-400 mg EGCG per day taken with food), most people tolerate it well. But at higher doses, and particularly under certain conditions, it carries a real risk of liver injury that has been extensively documented [4][5][7].

Common side effects (at standard doses):

• Nausea and stomach discomfort, especially when taken on an empty stomach
• Headaches
• Dizziness
• Sleep disruption (from caffeine content)
• Increased urination

Serious safety concern: liver injury.
This is not a theoretical risk. Over 100 clinically apparent cases of liver injury have been documented in the medical literature, and NIH's LiverTox database assigns green tea extract a Likelihood Score of A, which is the highest certainty rating for supplement-associated liver injury [5]. The risk increases substantially at doses of 800 mg EGCG per day or above, where clinical trials have shown statistically significant elevations in liver enzymes (ALT and AST) [4][5].

The liver injury mechanism appears to involve both a dose-dependent component and an unpredictable, individual-susceptibility component. Research has identified a genetic marker, HLA allele B*35:01 (found in 72% of green tea liver injury cases versus 11-15% of the general population), suggesting that some people are genetically predisposed to this reaction [5]. This means that even moderate doses can occasionally cause liver problems in susceptible individuals.

Key safety rules based on available evidence:

• Take with food, not on an empty stomach [4][5][7]
• Keep EGCG intake below 800 mg per day [4][5]
• Discontinue immediately if you develop abdominal pain, dark urine, or yellowing of the skin or eyes (jaundice) [7]
• Avoid green tea extract if you have pre-existing liver conditions [7]
• Be cautious about combining with caloric restriction, as fasting may increase liver vulnerability [5]

The Science

Hepatotoxicity epidemiology and mechanism:
In the Minnesota Green Tea Trial (n=1,075 postmenopausal women, 1,315 mg total catechins daily for 12 months), ALT elevations occurred in 6.7% of the GTE group versus 0.7% of controls [5]. Among those who restarted after initial elevation, recurrent elevations occurred with shorter latency, consistent with an immune-mediated (idiosyncratic) mechanism rather than purely dose-dependent toxicity [5].

The proposed mechanisms involve EGCG-induced mitochondrial toxicity and reactive oxygen species generation in hepatocytes [5]. The strong HLA B*35:01 association (OR approximately 20) supports an immunologic etiology, analogous to HLA-associated drug-induced liver injury seen with other compounds [5]. Liver injury patterns are predominantly hepatocellular (62.5%), with cholestatic (18.75%) and mixed (18.75%) patterns also observed [6].

Case presentations typically feature marked aminotransferase elevations (often >20x ULN), minimal alkaline phosphatase increases, negative viral hepatitis serology, and biopsy findings of necrosis, inflammation, and eosinophils [5]. Most cases resolve within 1-2 months of discontinuation, but acute liver failure requiring transplantation and fatal outcomes have been documented [5].

EFSA safety assessment:
The EFSA Panel on Food Additives and Nutrient Sources concluded that green tea catechins from traditional infusions are generally considered safe, but concentrated supplements at or above 800 mg EGCG/day cause elevated serum transaminases indicative of liver injury [4]. A safe intake level of 338 mg EGCG/day was derived for solid bolus supplement forms, while an observed safe level of 704 mg EGCG/day was considered for beverage forms [4].

Reproductive safety:
No teratogenic effects observed at 1000 mg/kg/day in animal studies. No genotoxicity at doses up to 2000 mg/kg orally. Not classified as a carcinogen [6]. However, caffeine content warrants caution during pregnancy and breastfeeding [7].

Dosing & Usage Protocols

The Basics

Green tea extract dosing requires balancing effectiveness with safety, and the margin between a potentially beneficial dose and one that raises liver concerns is narrower than with many supplements.

Commonly reported supplemental dose ranges, organized by research context:

Important dosing considerations based on available data:

• EFSA's derived safe intake is 338 mg EGCG/day from supplement forms [4], which is lower than the effective dose range cited in many clinical studies
• Most green tea extract products contain approximately 50% EGCG by weight, so a "500 mg green tea extract" capsule typically delivers about 250 mg of EGCG
• Split dosing (e.g., 200 mg EGCG twice daily rather than 400 mg at once) may reduce peak plasma concentrations and liver burden, though this has not been formally validated in safety studies
• Caffeine content varies significantly between products (0 to ~50 mg per capsule); caffeine-sensitive individuals or those taking other stimulants should choose decaffeinated formulations

The evidence suggests that the fat-burning effects of EGCG are synergistic with caffeine but substantially diminished in habitual caffeine consumers [1]. For individuals who regularly drink coffee or other caffeinated beverages, the metabolic boost from green tea extract may be minimal.

The Science

Dose-response relationships:
Doses of 400-500 mg EGCG daily represent the most commonly cited effective range across clinical studies for metabolic outcomes [1][6]. However, significant inter-study variability exists, with some trials showing significant weight and BMI reductions at doses below 500 mg over 12 weeks [6], while others found no benefit from 300 mg/day for 12 weeks in the context of caloric restriction.

Pharmacokinetic considerations:
Peak plasma concentrations are achieved at 1.0-2.2 hours post-ingestion [6]. The half-life of 1.9-4.6 hours suggests that twice-daily dosing maintains more consistent plasma levels than single daily doses. However, the hepatotoxicity risk at cumulative daily doses above 800 mg EGCG may outweigh pharmacokinetic advantages of split dosing at higher total doses [4][5].

Single-dose tolerance:
Single doses up to 1.6 grams have been reported as well-tolerated acutely [5], but repeated daily dosing at high levels carries progressive hepatotoxic risk. The maximum tolerated dose in clinical settings has been reported as high as 9.9 grams daily (equivalent to approximately 24 cups of tea), though this represents acute tolerance rather than safe chronic dosing [5].

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What to Expect (Timeline)

Green tea extract is not a fast-acting supplement for most outcomes. The timeline varies considerably depending on what you are hoping to achieve and your individual starting point.

Week 1-2:
Most users notice the caffeine-related effects first: improved alertness, mild energy boost, and possibly some GI adjustment (mild nausea or stomach discomfort if not taken with food). These are the most immediate, noticeable changes and are largely attributable to the caffeine and L-theanine content rather than EGCG specifically. Some people also notice increased urination frequency.

Week 3-4:
If present, GI side effects typically resolve as your body adjusts. Some users report subtle improvements in mental clarity and sustained energy without crashes. Fat oxidation effects, if they are going to occur, may begin during this period, though they are difficult to detect without formal testing.

Week 5-8:
This is the timeframe where metabolic effects begin to show in clinical trial data. Improvements in fasting blood glucose, lipid profiles, and inflammatory markers have been documented at the 8-week mark in controlled studies [6]. Weight changes, if any, are typically modest (a few pounds at most) and most apparent when combined with exercise and dietary management.

Week 8-12+:
Longer-term cardiovascular benefits, including blood pressure reductions and sustained lipid improvements, are most consistently seen in trials lasting 8 to 12 weeks or longer [6]. The antioxidant and anti-inflammatory effects accumulate gradually, and their impact on overall health is generally measured through biomarker testing rather than subjective experience.

What NOT to expect:
Green tea extract is unlikely to produce dramatic, immediately noticeable changes in body composition, energy, or health markers. The benefits are incremental and work best as part of a broader health strategy. Users who expect rapid weight loss from green tea extract alone are frequently disappointed, as the evidence supports only modest supplementary effects.

Interactions & Compatibility

SYNERGISTIC

• Caffeine: EGCG and caffeine work synergistically for fat oxidation and thermogenesis. Most green tea extracts naturally contain caffeine. The combination enhances energy expenditure more than either compound alone, though this effect is substantially reduced in habitual caffeine consumers.
• Vitamin C: May enhance catechin stability in the GI tract and improve absorption. Ascorbic acid has been shown to protect EGCG from oxidative degradation at intestinal pH.
• Quercetin: Both are polyphenolic compounds with overlapping antioxidant and anti-inflammatory pathways. Quercetin may enhance EGCG bioavailability through inhibition of COMT-mediated methylation.
• L-Theanine: Naturally present in green tea. The caffeine-theanine combination produces "alert calm" by modulating alpha brain wave activity. Taking L-theanine alongside caffeinated green tea extract may enhance cognitive benefits while reducing caffeine-induced jitteriness.
• Fish Oil (EPA/DHA): Complementary cardiovascular support through different mechanisms. Fish oil addresses triglycerides and inflammatory pathways, while green tea extract addresses LDL oxidation and endothelial function.
• Turmeric/Curcumin: Both compounds inhibit NF-kB signaling and provide anti-inflammatory effects through complementary pathways. No known negative interactions.

CAUTION / AVOID

• Iron: EGCG may reduce non-heme iron absorption in a dose-dependent manner. Individuals with iron deficiency or those taking iron supplements should separate green tea extract and iron by at least 2 hours.
• Vitamin B9 (Folate): EGCG inhibits dihydrofolate reductase (DHFR), potentially reducing folic acid bioavailability. This interaction is particularly relevant during pregnancy and for individuals with megaloblastic anemia.
• Bortezomib (Velcade): EGCG directly inhibits the therapeutic effect of this proteasome inhibitor chemotherapy agent. Avoid green tea extract during bortezomib treatment.
• Irinotecan: EGCG inhibits UGT1A1-mediated transport of the active metabolite SN-38, prolonging its half-life and increasing toxicity risk.
• Nadolol: EGCG inhibits the OATP1A2 transporter, reducing nadolol absorption and potentially diminishing its blood pressure-lowering effect.
• Rosuvastatin and other statins: Green tea extract has been shown to significantly lower systemic exposure of rosuvastatin, potentially reducing its therapeutic efficacy.
• Verapamil: EGCG increases verapamil bioavailability via P-glycoprotein inhibition, potentially increasing side effects.
• Warfarin and anticoagulants: Very high green tea intake (0.5-1 gallon daily) may antagonize anticoagulant effects due to vitamin K content. Standard supplement doses are unlikely to cause clinically significant interactions, but monitoring is recommended.
• CYP3A4 substrate medications: EGCG can modulate CYP3A4 activity, potentially affecting the metabolism of a wide range of medications processed by this enzyme pathway.
• NAC (N-Acetyl Cysteine): Theoretically, NAC's antioxidant properties could interfere with EGCG's pro-oxidant anticancer mechanisms. Clinical significance is uncertain, but oncology patients using EGCG therapeutically should discuss this with their treatment team.

How to Take / Administration Guide

Recommended forms:
Standardized green tea extract capsules with a specified EGCG percentage (typically 45-50%) offer the most consistent and predictable dosing. Look for products that clearly state the EGCG content per serving rather than just "total catechins" or "total polyphenols," since these broader categories make it difficult to know how much EGCG you are actually consuming.

Decaffeinated formulations are available for individuals sensitive to caffeine or those taking green tea extract in the afternoon or evening. Be aware that decaffeination processes can reduce catechin content, so verify the EGCG content on the label.

Timing considerations:

• Take with food. This is the most important practical guideline for green tea extract. Taking it on an empty stomach increases both absorption and hepatotoxicity risk. Both EFSA and USP recommend consumption with food [4][5][7].
• If taking a caffeinated formulation, avoid late afternoon or evening doses to minimize sleep disruption.
• Separate from iron supplements by at least 2 hours if iron status is a concern.
• If splitting the daily dose, morning and early afternoon with meals are the most common timing.

Stacking guidance:

• Pairs well with caffeine (if not already present in the extract) for enhanced metabolic effects.
• Can be taken alongside most other antioxidant supplements without known adverse interactions.
• Separate from iron and folic acid supplements by 2+ hours.
• If taking medications, review the Interactions section carefully, particularly for cardiovascular drugs, chemotherapy agents, and CYP3A4 substrates.

Cycling guidance:
There is no established evidence-based cycling protocol for green tea extract. Some practitioners suggest periodic breaks (e.g., 4-6 weeks on, 1-2 weeks off) to reduce cumulative hepatotoxic burden, but this approach has not been validated in clinical studies. Individuals using green tea extract long-term may consider periodic liver function testing (ALT, AST) as a monitoring strategy.

Choosing a Quality Product

Green tea extract is one of the supplement categories where product quality varies enormously, and this variation has direct safety implications. Testing of 97 dietary supplements found that 50% contained catechins, with concentrations varying from 1 microgram to 486 mg per gram, and 40% of products did not declare catechin content on their labels [5].

Third-party certifications to look for:

• USP Verified: Confirms identity, strength, purity, and proper disintegration. USP has also issued specific safety guidance for green tea extract, recommending consumption with food.
• NSF Certified for Sport: Screens for banned substances. Important for athletes concerned about contamination.
• ConsumerLab Approved: Independent testing for catechin content accuracy and purity.
• Informed Sport: Batch testing for WADA-banned substances.

What to look for on the label:

• Specific EGCG content per serving (not just "total polyphenols" or "catechins")
• Standardization percentage (e.g., "standardized to 50% EGCG")
• Whether the product is caffeinated or decaffeinated, with caffeine amount specified
• Serving size clarity (some products list total green tea extract while others list EGCG content)
• Manufacturing location and GMP compliance

Red flags:

• Products that do not specify EGCG content or standardization percentage
• Mega-dose products (>800 mg EGCG per serving) without clear safety warnings
• "Proprietary blend" formulations that obscure individual ingredient amounts
• Weight loss or "fat burner" products that combine green tea extract with multiple stimulants, increasing both caffeine load and potential hepatotoxic stress
• Products marketed with unsubstantiated therapeutic claims (cancer cure, rapid weight loss)

Form quality hierarchy:
Standardized extract capsules with declared EGCG percentage offer the most reliable dosing. Unstandardized whole-leaf powders and liquid extracts may have less predictable catechin content per serving.

Storage & Handling

EGCG is sensitive to heat, light, oxygen, and moisture. Proper storage helps maintain catechin potency over the product's shelf life.

• Store in a cool, dry place away from direct sunlight. Temperatures below 25 degrees C (77 degrees F) are ideal.
• Keep the container tightly sealed between uses. Exposure to air accelerates EGCG oxidation.
• Avoid storing in bathrooms or near stovetops where heat and humidity fluctuate.
• Liquid green tea extracts may require refrigeration after opening; follow manufacturer instructions.
• Powder forms are more susceptible to moisture absorption. If using bulk powder, consider storing in an airtight container with a desiccant packet.
• The "creaming down" phenomenon (visible when brewed tea cools) precipitates up to 83% of EGCG, which is relevant for liquid preparations. This does not apply to capsules or tablets.

Lifestyle & Supporting Factors

Dietary sources of green tea catechins:
Green tea is the richest dietary source of EGCG. Other sources include white tea (similar catechin profile but lower EGCG concentration), oolong tea (partial oxidation reduces catechin content), and small amounts in certain fruits (apples, berries). Black tea undergoes full oxidation, converting most catechins to theaflavins and thearubigins, resulting in minimal EGCG content.

A typical cup of brewed green tea provides approximately 50 mg of EGCG, meaning that achieving supplement-equivalent doses (300-500 mg/day) through tea alone would require 6 to 10 cups daily.

Factors that may increase the need for antioxidant support:

• High levels of physical exercise (increases oxidative stress)
• Environmental pollutant exposure
• Chronic psychological stress
• Diets low in fruits, vegetables, and whole grains
• Smoking (dramatically increases free radical burden)
• Excessive alcohol consumption (increases oxidative and hepatic stress; note that alcohol plus green tea extract may compound liver risk)

Exercise considerations:
Green tea extract, particularly when combined with caffeine, may modestly enhance fat oxidation during exercise [1][6]. Some clinical trials have shown enhanced exercise-induced abdominal fat loss in overweight adults taking green tea extract. However, this effect is diminished in regular caffeine consumers.

Sleep and caffeine management:
If using a caffeinated green tea extract formulation, timing matters for sleep quality. Caffeine's half-life is approximately 5-6 hours, so taking green tea extract after mid-afternoon may interfere with sleep onset. Consider decaffeinated formulations if sleep sensitivity is a concern.

Monitoring recommendations:
For individuals using green tea extract at doses above 300 mg EGCG daily, or for extended periods, periodic liver function testing (ALT, AST) is a prudent monitoring strategy. This is particularly relevant for individuals with any history of liver sensitivity, alcohol use, or concurrent use of other potentially hepatotoxic medications or supplements.

Regulatory Status & Standards

United States (FDA):
Green tea extract is regulated as a dietary supplement under DSHEA. It is not approved for any specific medical indication and is not evaluated by the FDA for efficacy. Green tea and its catechins have Generally Recognized as Safe (GRAS) status as food ingredients, but concentrated supplement forms occupy a different regulatory space. The FDA has not established an upper intake level for EGCG from supplements. NIH LiverTox assigns a Likelihood Score of A (well-established cause of liver injury) [5]. The product Exolise (standardized green tea extract) was withdrawn from markets in Spain and France in 2003 following liver injury reports.

European Union (EFSA):
EFSA conducted a comprehensive safety assessment of green tea catechins, concluding that catechins from traditional tea infusions are generally safe, but concentrated supplements at or above 800 mg EGCG/day cause liver enzyme elevations [4]. A safe intake level of 338 mg EGCG/day was derived for supplement forms. Green tea extract supplements are permitted in the EU but must comply with maximum levels set by individual member states.

Canada (Health Canada):
Green tea extract is available as a Natural Health Product (NHP). Licensed products carry a Natural Product Number (NPN). Health Canada monographs provide dosing guidance and require appropriate safety warnings on labels.

Australia (TGA):
Green tea extract is available as a Listed Medicine. The TGA regulates it as a complementary medicine with specific compositional and labeling requirements.

Athlete & Sports Regulatory Status:

• WADA: Green tea extract and EGCG are not on the WADA Prohibited List. A clinical study confirmed that green tea consumption does not distort steroid profile parameters used in doping controls, though in vitro data had suggested possible UGT2B17 inhibition affecting testosterone glucuronidation.
• USADA: Green tea extract is not listed on the USADA Supplement Connect High Risk List. USADA advises athletes that natural product supplements are used at the athlete's own risk regarding contamination.
• NCAA: Not on the NCAA banned substance list. NCAA athletic departments should provide supplements that carry NSF Certified for Sport or Informed Sport certification.
• GlobalDRO: Athletes can verify the current status of green tea extract products at GlobalDRO.com.
• Certification programs: NSF Certified for Sport and Informed Sport-certified green tea extract products are available, providing additional assurance against contamination with prohibited substances.

Regulatory status and prohibited substance classifications change frequently. Athletes should always verify the current status of any supplement with their sport's governing body, their national anti-doping agency, and a qualified sports medicine professional before use. Third-party certification (Informed Sport, NSF Certified for Sport) reduces but does not eliminate the risk of contamination with prohibited substances.

Frequently Asked Questions

Is green tea extract safe for daily use?
Based on available evidence, green tea extract at doses within the EFSA-derived safe intake of 338 mg EGCG per day, taken with food, appears to be well-tolerated for most adults. However, long-term safety data remains limited, and individual susceptibility to liver effects varies. Some practitioners suggest periodic liver function testing for long-term users, particularly at higher doses. People with pre-existing liver conditions should avoid green tea extract supplements.

How much EGCG is in a cup of green tea versus a supplement?
A typical cup of brewed green tea contains approximately 50 mg of EGCG, though this varies significantly with brewing time, temperature, and tea quality (range: 2.3 to 203 mg per 100g of brewed tea). Supplement capsules typically contain 200 to 500 mg of EGCG per serving. Achieving supplement-equivalent doses through tea alone would require 6 to 10 cups daily.

Can green tea extract cause liver damage?
Yes, this is a well-documented risk. Over 100 clinically apparent cases of liver injury have been reported in the medical literature, and green tea extract has been identified as a leading cause of herbal supplement-associated liver injury. The risk increases substantially at doses of 800 mg EGCG per day or above, when taken on an empty stomach, and in genetically susceptible individuals (HLA B*35:01 carriers). Most cases resolve upon discontinuation, but acute liver failure requiring transplantation has been documented.

Should I take green tea extract on an empty stomach?
Based on available safety data, no. Both EFSA and USP recommend taking green tea extract with food. While fasting increases EGCG absorption, it also increases hepatotoxicity risk. The recommendation to take with food prioritizes safety over maximal absorption.

Does green tea extract help with weight loss?
Clinical evidence supports modest fat oxidation and weight management effects, particularly at doses of 400-500 mg EGCG daily when combined with exercise. However, the effects are small (typically a few pounds over several months), inconsistent across studies, and substantially diminished in habitual caffeine consumers. Green tea extract is not effective as a standalone weight loss solution.

Can I take green tea extract with other medications?
Green tea extract has documented interactions with multiple medication classes, including certain chemotherapy agents (bortezomib, irinotecan), cardiovascular drugs (nadolol, rosuvastatin, verapamil), and anticoagulants. It also modulates CYP3A4 enzyme activity, which can affect the metabolism of many medications. Anyone taking prescription medications should consult with a healthcare provider or pharmacist before adding green tea extract.

Is decaffeinated green tea extract as effective?
Decaffeinated formulations retain EGCG and other catechins but lose the caffeine-EGCG synergy that contributes to fat oxidation and thermogenic effects. For antioxidant and cardiovascular benefits that are primarily catechin-driven, decaffeinated products are likely comparable. For metabolic and weight management goals, the caffeinated form may offer additional benefit, though this advantage is diminished in habitual caffeine consumers.

Is green tea extract safe during pregnancy?
Available animal data shows no teratogenic effects at high doses, and EGCG itself has not demonstrated genotoxicity. However, the caffeine content in most green tea extract products is a concern during pregnancy, and EGCG's inhibition of dihydrofolate reductase (DHFR) may reduce folic acid bioavailability, which is critical during fetal development. Most clinical guidelines advise pregnant women to limit caffeine intake and to prioritize folic acid supplementation, making green tea extract a supplement that warrants discussion with a healthcare provider before use during pregnancy.

How long does it take to see benefits from green tea extract?
Based on available clinical data, cardiovascular and metabolic improvements typically require 8 to 12 weeks of consistent use to manifest in measurable ways. Energy and alertness effects from the caffeine component are immediate. Weight management effects, when present, are gradual and modest.

What is the difference between green tea extract and matcha?
Matcha is powdered whole green tea leaf, consumed in suspension rather than infusion. It contains EGCG and other catechins along with the leaf fiber, chlorophyll, and full caffeine content. Green tea extract is a concentrated, standardized extract of the leaf. Gram for gram, standardized green tea extract delivers significantly more EGCG than matcha powder, but matcha provides the full spectrum of tea leaf compounds including higher L-theanine content.

Myth vs. Fact

Myth: Green tea extract is a powerful fat burner that causes significant weight loss.
Fact: Clinical evidence shows modest fat oxidation effects, with most studies reporting small reductions in body weight (1-2 kg over 8-12 weeks) that are statistically significant but practically modest [1][6]. The effects are further diminished in habitual caffeine consumers. Green tea extract may provide a small supplementary benefit alongside diet and exercise but is not an effective standalone weight loss intervention.

Myth: Green tea extract supplements are as safe as drinking green tea.
Fact: EFSA's scientific assessment explicitly distinguishes between traditional tea infusions (generally safe) and concentrated supplement forms (not considered safe at high doses) [4]. The concentrated bolus delivery of EGCG in supplement form creates a different pharmacokinetic and safety profile. Over 100 cases of liver injury have been attributed to green tea extract supplements, while drinking brewed green tea has not been associated with the same level of hepatotoxic risk [5].

Myth: More EGCG is always better.
Fact: EGCG has a well-documented dose-response ceiling beyond which risks increase without proportional benefit gains. Doses at or above 800 mg EGCG daily are associated with elevated liver enzymes [4][5]. At the molecular level, high concentrations of EGCG shift from antioxidant to pro-oxidant activity, generating the same reactive oxygen species it neutralizes at lower concentrations [2][3].

Myth: Green tea extract can prevent or treat cancer.
Fact: While EGCG demonstrates impressive anticancer activity in cell culture and animal studies across multiple cancer types, clinical translation to humans remains limited and inconsistent [2][7]. A prostate cancer prevention trial actually showed elevated incidence with supplementation [7]. Preclinical promise should not be confused with proven clinical efficacy. Green tea extract should not be used as a substitute for evidence-based cancer treatments.

Myth: All green tea extract products contain the same amount of EGCG.
Fact: Product variability is a significant quality concern. Independent testing found catechin concentrations in supplements ranging from 1 microgram to 486 mg per gram, and 40% of tested products did not declare catechin content on their labels [5]. Choosing a product with specific EGCG standardization and third-party testing is essential for predictable dosing.

Myth: Green tea extract should be taken on an empty stomach for maximum benefit.
Fact: While fasting does increase EGCG absorption, both EFSA and USP recommend taking green tea extract with food because fasting also increases the risk of liver injury [4][5][7]. The safety trade-off clearly favors consumption with food.

Myth: Green tea extract provides the same benefits as drinking green tea.
Fact: Green tea extract delivers concentrated catechins without the full spectrum of compounds found in brewed tea, including certain amino acids, minerals, and the modulating effects of hot water extraction. Conversely, achieving clinical-dose EGCG levels through tea alone would require consuming 6-10 cups daily, which is impractical for most people. The two forms complement but do not perfectly substitute for each other.

Sources & References

Systematic Reviews & Meta-Analyses

[1] Hu J, Webster D, Cao J, Shao A. The safety of green tea and green tea extract consumption in adults: Results of a systematic review. Regul Toxicol Pharmacol. 2018;95:412-433. PubMed: 29580974. https://pubmed.ncbi.nlm.nih.gov/29580974/

Government & Institutional Sources

[4] EFSA Panel on Food Additives and Nutrient Sources. Scientific opinion on the safety of green tea catechins. EFSA Journal. 2018;16(4):e05239. PMC7009618. https://pmc.ncbi.nlm.nih.gov/articles/PMC7009618/

[5] National Institute of Diabetes and Digestive and Kidney Diseases. LiverTox: Clinical and Research Information on Drug-Induced Liver Injury. Green Tea entry. National Library of Medicine, Bethesda, MD. NBK547925. https://www.ncbi.nlm.nih.gov/books/NBK547925/

Clinical Evidence & Reviews

[2] Singh BN, Shankar S, Srivastava RK. Green tea catechin, epigallocatechin-3-gallate (EGCG): mechanisms, perspectives and clinical applications. Biochem Pharmacol. 2011;82(12):1807-1821. PMC4082721. https://pmc.ncbi.nlm.nih.gov/articles/PMC4082721/

[3] Khan N, Mukhtar H. Tea polyphenols in promotion of human health. Nutrients. 2018;11(1):39. Epigallocatechin-3-gallate (EGCG): Chemical and biomedical perspectives. Phytochemistry. 2006;67(19):2318-2330. PMC2903211. https://pmc.ncbi.nlm.nih.gov/articles/PMC2903211/

[6] Almatroodi SA, Alrumaihi F, Alsahli MA, Alhommrani MF, Khan A, Rahmani AH. Epigallocatechin Gallate (EGCG): Pharmacological Properties, Biological Activities and Therapeutic Potential. Nutrients. 2025. PMC11821029. https://pmc.ncbi.nlm.nih.gov/articles/PMC11821029/

[7] Memorial Sloan Kettering Cancer Center. Green Tea monograph. Integrative Medicine, About Herbs database. https://www.mskcc.org/cancer-care/integrative-medicine/herbs/green-tea

Additional Primary Sources Referenced

[8] Mazzanti G, et al. Hepatotoxicity of green tea: an update. Arch Toxicol. 2015;89(8):1175-1191.

[9] Younes M, et al. Scientific opinion on the safety of green tea catechins. EFSA Journal. 2018;16(4):e05239.

[10] Dostal AM, et al. Green tea extract and catechol-O-methyltransferase genotype modify fasting serum insulin and plasma adiponectin concentrations in a randomized controlled trial of overweight and obese postmenopausal women. J Nutr. 2016;146(1):38-45.

[11] Inoue M, et al. Regular consumption of green tea and the risk of breast cancer recurrence. Cancer Lett. 2001;167(2):175-182.

[12] Pisters KM, et al. Phase I trial of oral green tea extract in adult patients with solid tumors. J Clin Oncol. 2001;19(6):1830-1838.

[13] Hu J, et al. The safety of green tea and green tea extract consumption in adults: Results of a systematic review. Regul Toxicol Pharmacol. 2018;95:412-433. PubMed: 29580974.

[14] Church RJ, et al. Hepatotoxicity with high-dose green tea extract: effect of catechol-O-methyltransferase and uridine 5'-diphospho-glucuronosyltransferase 1A4 genotypes. PMC10060436. https://pmc.ncbi.nlm.nih.gov/articles/PMC10060436/

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