Anastrozole: The Definitive Guide

What is anastrozole?

Anastrozole is a prescription tablet that lowers estrogen by blocking the enzyme that makes it. It is approved only to treat breast cancer in women after menopause. Men meet it in two very different ways: added to testosterone therapy to bring down a high estradiol result, or on its own to raise their own testosterone. [1], [2], [3]

It belongs to a group called aromatase inhibitors. Aromatase is the enzyme that converts testosterone into estradiol, the main form of estrogen. It is found in fat tissue, the testicles, the brain and bone. Anastrozole sits in that enzyme and stops it working for as long as the drug is present, so less estradiol is made. It is not a steroid or a hormone; the US label calls it a selective non-steroidal aromatase inhibitor. [4], [1]

Two facts surprise many men on TRT. Men need estradiol for strong bones, body-fat control and sex drive, so lower is not automatically better. And no label or major guideline gives a dose for adding anastrozole to TRT; the US Veterans Affairs (VA) recommendations do not generally recommend it. [5], [6], [7]

It was first approved in the United States on December 27, 1995, as Arimidex. [8]

[9], [10], [11], [12], [13], [14]

1 mg is the only strength sold anywhere covered here, and there is no licensed liquid. The tablets are not scored for splitting (see tablets). [1], [10]

Typical Anastrozole Protocols

Every label, trial and study uses the 1 mg tablet, most often once a day. The two guideline ranges are for anastrozole used instead of testosterone, not added to it. No label or major guideline gives a dose for adding anastrozole to TRT; the only published on-TRT clinic schedule is 0.5 mg three times a week (below). [1], [15], [2], [3]

Titration Adjusting the dose

The labels do not adjust the dose. When anastrozole is used instead of testosterone, the AUA adjusts it by estradiol to keep estradiol in range, stops it if estradiol stays high, and checks blood no sooner than 4 weeks after starting. The one published on-TRT clinic series changed the dose or frequency to keep estradiol between 20 and 40 pg/mL (about 73–147 pmol/L). No guideline sets an estradiol target for men on TRT. [15], [3]

Breaks and courses

Anastrozole is not a cycle and has no planned breaks. The AUA says aromatase inhibitors should in general not be used for long periods because of bone loss, while the Brazilian statement describes continuous use while it is needed and tolerated, with long-term follow-up. Stopping is covered below. [15], [2]

Read the units. Anastrozole comes only as a 1 mg tablet, so any other dose means taking it less often or cutting tablets. The tablets are not scored, and no label gives splitting instructions, so a "quarter tablet" is not a measured 0.25 mg. No syringe arithmetic applies. Compare weekly totals: 1 mg twice a week is 2 mg a week; 1 mg daily is 7 mg. [1], [10], [11]

These are labeled, guideline and study doses; no label or major guideline gives a dose for adding anastrozole to TRT, and prescribers who use it adjust by estradiol and symptoms. These guides are for men; testosterone therapy for women is covered in testosterone therapy for women.

Cautions for every schedule above. Lowering estradiol lowers bone density in men, and the labels advise considering bone and cholesterol checks. The Brazilian statement rules it out with osteoporosis, marked bone thinning (osteopenia) or severe liver disease. The labels say it should not be taken with tamoxifen or estrogen-containing medicines, and taking it alongside TRT does not protect sperm production (fertility). For breast symptoms on TRT, the AUA considers lowering the testosterone dose first (where sources differ). [22], [1], [11], [2], [23], [24]

Where schedules differ: whether to use it on TRT at all, which estradiol level, lowering testosterone first, daily vs twice weekly ↓

What do clinic protocol sources use on TRT?

This is the only complete, published on-TRT schedule found for this guide (the paper's "testosterone therapy" group also included some men on clomiphene). Median estradiol fell from 65 to 22 pg/mL (about 239 to 81 pmol/L) while testosterone did not change (616 vs 596 ng/dL, about 21.4 vs 20.7 nmol/L). Symptoms, bone and side effects were not reported. [3]

Elsewhere, practice varies widely: in a 2020 survey of 489 sexual-medicine clinicians, anastrozole starting doses ranged from 1 mg a week to 1 mg a day, a sevenfold spread of habits rather than a schedule. [33]

What is anastrozole commonly used for in men?

Its only labeled use is breast cancer in women after menopause; the UK label adds breast cancer prevention in women at higher risk. Everything below is off-label use in men, and the evidence behind each use differs a great deal. [1], [11]

• Lowering estradiol on TRT. The most common reason men on testosterone hear about it. Some clinics prescribe it for a lab number, some for symptoms, some for everyone. The evidence for any benefit here is thin.
• Raising testosterone instead of TRT. Blocking estradiol makes the brain send more signal to the testicles, so a man whose testicles work makes more testosterone. Testosterone rises reliably; how men feel does not.
• Fertility. For men with low testosterone who want children, several guidelines list aromatase inhibitors as an off-label alternative to testosterone, which shuts sperm production down.
• Breast growth (gynecomastia). Anastrozole has done poorly in the trials that tested it; tamoxifen did better.
• Specialist uses. Male breast cancer (always with a second drug), some pituitary tumors and rare genetic conditions.

[3], [16], [24], [34], [35]

Does anastrozole help men on TRT?

It lowers estradiol, but no trial has shown that adding it to TRT makes men feel or function better. On full-dose TRT the brain's signal to the testicles is largely switched off, so in the one clinic series testosterone did not change when anastrozole lowered estradiol; with waning pellets, adding it kept testosterone up longer. [3], [21]

• The only randomized trial found in men treated for low testosterone gave 40 men with epilepsy, low testosterone and low sex drive testosterone injections plus anastrozole or a placebo for 3 months. Sexual function returned to normal in 72.2% vs 47.4%, a difference that was not statistically significant. [31]
• A research trial in 60 healthy older men gave testosterone enanthate 100 mg weekly with or without anastrozole 1 mg daily for 6 weeks. Testosterone alone raised estradiol by 81%; adding anastrozole cut it by 50%. Verbal memory improved only in the men whose estradiol rose. [20]
• In men on testosterone pellets, adding 1 mg daily kept estradiol lower and made pellets last longer (about 198 vs 128 days), in a records review with no bone or symptom outcomes. [21]
• Estradiol and sex drive. In the Testosterone Trials of older men, larger rises in estradiol, as well as testosterone, went with larger gains in sexual desire and activity. When researchers lowered men's hormones on purpose, missing estradiol mainly caused fat gain, and both hormones contributed to sexual function. The International Consultation on Sexual Medicine suggests not using aromatase inhibitors to treat low desire. [36], [5], [37]

How often is estradiol high on TRT? In records from one US men's clinic chain that treated about half of its men with injections, 7,215 of 34,016 men (20.2%) had estradiol at or above 42.6 pg/mL (about 156 pmol/L), measured by a standard immunoassay; the figure mixes men who were screened and men who were treated. High estradiol was not linked to low libido in that data, and the clinic prescribed estrogen-blocking drugs regardless of the level. [38]

Can anastrozole raise testosterone instead of TRT?

Yes, in men whose testicles work, but the trials found no clear benefit from the higher number: no better quality of life or erections at 12 weeks, and no change in body composition or strength after a year. By lowering estradiol, anastrozole lets the pituitary send more LH, and working testicles make more testosterone. [28], [16]

• In 37 men aged 62 to 74, 12 weeks of 1 mg daily raised total testosterone from 343 to 572 ng/dL (about 12 to 20 nmol/L); 1 mg twice a week raised it from 397 to 520 ng/dL (about 14 to 18 nmol/L). Estradiol fell from about 26 to 17 pg/mL in both groups. Quality of life, erections, urinary symptoms and hematocrit did not change. [16]
• In 88 men aged 60 and older, a year of 1 mg daily raised testosterone from 11.2 to 18.2 nmol/L (about 323 to 525 ng/dL) at 3 months, but body composition and strength did not change. In the same trial's bone study, spine bone density fell (bone). [17], [22]
• In 43 men aged 65 to 82 given testosterone gel, anastrozole or placebo for 12 months, both raised testosterone above 500 ng/dL (about 17 nmol/L), but only the gel improved spine bone density and fast walking speed; lean mass rose only with anastrozole (about 1.2–1.5 kg). [6]
• Related drugs tell the same story. An investigational aromatase inhibitor normalized testosterone in 271 men with obesity over 24 weeks without improving sexual symptoms or body composition, and letrozole raised testosterone from 8.6 to 21.5 nmol/L (about 248 to 620 ng/dL) in 42 men with obesity with no physical or psychological benefit. [39], [40]

It only works when the brain-to-testicle chain is intact; the European urology guideline says these drugs cannot work in primary hypogonadism (failing testicles) or when the pituitary is damaged. In a head-to-head trial of 26 infertile men with low testosterone, clomiphene 25 mg a day raised testosterone more than anastrozole 1 mg a day over 12 weeks; estradiol rose on clomiphene and fell on anastrozole, and neither improved semen or how men felt. [41], [18]

Does anastrozole help fertility?

For some men with low testosterone and low sperm counts, it improves hormones and sperm numbers, but the studies are small and mostly uncontrolled, and it does nothing for sperm while taking TRT. [42], [24]

Guidelines give it a limited, off-label place:

• The US male infertility guideline from the AUA and the American Society for Reproductive Medicine (AUA/ASRM) allows aromatase inhibitors, hCG (human chorionic gonadotropin, an LH-like hormone injection) or SERMs for infertile men with low testosterone (a conditional recommendation on weaker evidence), and notes an aromatase inhibitor may be considered when estradiol is also high. [24]
• The AUA, Canadian and British guidelines and the 2026 Brazilian statement list aromatase inhibitors among alternatives to testosterone for men who want to keep fertility. [15], [43], [44], [2]
• The European urology guideline says not to treat unexplained infertility routinely with aromatase inhibitors. [45]

What the studies found:

• Sperm counts rose in selected men. In 25 men with low counts, sperm concentration rose from 5.5 to 15.6 million per mL on 1 mg daily; in 30 men with overweight, from 7.8 to 14.2 million per mL, and 14 couples became pregnant, 10 of them through in vitro fertilization (IVF). Neither study had a control group. [46], [19]
• Men with no sperm rarely respond. In one series, 18 of 21 men with low counts improved but none with azoospermia did, and a 2023 study of 90 men (median dose 3 mg a week) found 41 moved up a sperm-count category, 11 moved down and 26 reached normal counts, with the best results in men who had some sperm and a high testosterone-to-LH ratio. [47], [30]
• Reviews are cautious. A 2020 review of 8 studies (only two small placebo-controlled trials, neither of anastrozole) called for larger placebo-controlled trials; a 2022 meta-analysis of 10 studies found no advantage over SERMs or hCG; a 2024 analysis of 14 randomized trials found insufficient evidence for routine use. [42], [48], [49]

Taken with TRT, it does not protect sperm. The AUA/ASRM guideline says men who want children now or later should not be prescribed testosterone, and calls the evidence for adding other medicines to testosterone too limited to recommend. See TRT and fertility. [24]

Does anastrozole help gynecomastia (breast growth)?

It has done poorly in the trials that tested it. In a double-blind trial of 114 men taking the prostate cancer drug bicalutamide, breast growth developed in 73% on placebo, 51% on anastrozole 1 mg daily and 10% on tamoxifen 20 mg daily. A second report from the same program found that tamoxifen, but not anastrozole, significantly reduced breast growth and pain, whether used to prevent it or to treat it. In 80 boys with breast growth during puberty, 6 months of 1 mg daily worked no better than placebo (38.5% vs 31.4% responded). [34], [50], [51]

On TRT, the only published evidence is a case report of two men. The AUA watches first, then lowers the testosterone dose if levels are upper-normal, or considers an aromatase inhibitor if they are low or normal. The European Academy of Andrology says drug treatment of breast growth is not justified in general, and the Brazilian statement names tamoxifen for breast growth on TRT. See the tamoxifen guide. [52], [15], [53], [2]

What specialist uses does anastrozole have in men?

• Male breast cancer. Guidelines prefer tamoxifen. When an aromatase inhibitor is used, it is paired with a drug that shuts down the pituitary signal (a GnRH agonist); on its own, an aromatase inhibitor raises LH and testosterone in men. [54], [35], [28]
• Prolactin-secreting pituitary tumors (9 reported patients) and Klinefelter syndrome before sperm-retrieval surgery (sperm found in 10 of 22 men after anastrozole vs 4 of 21 after an hCG-type drug, not statistically significant). [55], [27]
• Boys with growth hormone deficiency. The trial authors reported a gain in predicted adult height, but the UK regulator's reading of the same 52-boy trial found no significant height benefit and more fractures, and the UK label says not to use it this way. [56], [11]

It also appears in forums about recovering after steroid cycles; the UK Society for Endocrinology calls the evidence for any such drug "limited and of low quality", and notes most men's hormones recover within about 12 months of stopping. [57]

How quickly does anastrozole work?

Estradiol falls within a day. In women, 1 mg cut estradiol by about 70% within 24 hours. In young males, 0.5 mg and 1 mg daily each cut it by about 50% over 10 days, so halving the dose did not halve the effect. When it is used instead of TRT, the AUA's first blood test comes no sooner than 4 weeks after starting. [1], [29], [15]

This guide does not teach how to judge symptoms blamed on estrogen at home. The Academy note below covers a structured way to think them through before any add-on medicine.

Where do anastrozole protocols differ?

Most of the disagreement is not about the dose. It is about whether anastrozole belongs in a TRT plan at all, and that depends on which guideline you read and whether a man has symptoms or only a lab number.

Should anastrozole be added to TRT at all?

The guidelines that address it lean against routine use; practice is split.

[7], [41], [15], [58], [2], [37], [33]

A prescription written for one high estradiol result, with no symptoms, sits outside what these guidelines describe; under the one clinic's published criteria, only 44 of 1,708 men qualified. [3]

Which estradiol level calls for treatment?

There is no agreed number. The AUA suggests referring a man to an endocrinologist if estradiol is persistently above 40 pg/mL (about 147 pmol/L) before treatment, and acts on breast symptoms during it. The clinic series treated above 60 pg/mL, or 40–60 with breast tenderness. A fertility series added anastrozole above 50 pg/mL or when the testosterone-to-estradiol ratio fell below 10. A 2025 review says a testosterone-to-estradiol ratio of 10 to 30 "seems" beneficial but that no best value has been established. [15], [3], [32], [59]

The test itself adds doubt. Standard direct immunoassays read estradiol 6% to 74% higher than the more accurate mass spectrometry method in men, and the Endocrine Society says routine methods cannot measure men's low estradiol accurately. Units trip people up too: 1 pg/mL is about 3.67 pmol/L, so 40 pg/mL is about 147 pmol/L. See estrogen on TRT for assays, symptoms and timing. [60], [61], [62]

Lower the testosterone dose first?

The AUA's own path starts there: when breast symptoms persist with high estradiol and testosterone is upper-normal, it lowers the testosterone dose before any aromatase inhibitor. The reasoning is simple arithmetic: estradiol is made from testosterone, so a smaller testosterone dose makes less of it. How injection frequency and form affect estradiol is covered in estrogen on TRT and TRT dosage. [15]

Daily, twice a week or less often?

In the trial of older men, 1 mg twice a week lowered estradiol as much as 1 mg daily (both to 17 pg/mL), with a smaller testosterone rise. In young males, 0.5 mg daily lowered estradiol about as much as 1 mg. No trial in men has tested 1 mg once a week, doses of 0.25 mg or less, or timing doses to injection days, and no study has checked how evenly a cut tablet splits. The 1-mg-weekly-to-1-mg-daily spread in the clinician survey reflects that gap. [16], [29], [33], [10]

Does a low testosterone-to-estradiol ratio predict who benefits?

For fertility, sources disagree. An early series chose men for treatment because their testosterone-to-estradiol ratio was low. A 2023 study of 90 men found that baseline estradiol and the estradiol-to-testosterone ratio did not predict whose sperm improved; a high testosterone-to-LH ratio and having some sperm to start with did. [46], [30]

Anastrozole or tamoxifen for breast symptoms?

They work differently: anastrozole lowers estradiol throughout the body, while tamoxifen leaves estradiol high and blocks its effect in breast tissue. In the head-to-head trial in men on bicalutamide, tamoxifen prevented far more breast growth (10% vs 51%). They should not be taken together (interactions). See the tamoxifen guide. [34], [23]

What happens when you stop anastrozole?

The drug clears within days, and estradiol comes back as it does. [1], [11]

• The drug itself. With a half-life of 40–50 hours, most of it is gone within about a week. In women, estradiol stayed suppressed for up to 6 days after stopping daily 1 mg. No study has measured this in men on TRT. [11], [1]
• Estradiol. It returns toward the level set by the testosterone dose. No study has tested whether a rebound, a taper or a set recheck date after stopping is needed; labels give no tapering instructions. [1]
• Testosterone, when anastrozole was used instead of TRT. The trials measured the rise only while men were taking it; no study has followed men after stopping. [17], [16]

For stopping testosterone itself, see stopping TRT.

How is anastrozole taken, and how long does it last?

Anastrozole is swallowed. It comes only as a 1 mg tablet, it can be taken with or without food, and one dose keeps working for days. [1]

How long does anastrozole last in the body?

In women, blood levels peak about 2 hours after a dose; in a small study of 18 healthy men given a single 1 mg dose, the peak came at about 1.5 hours. Food lowers the peak by 16% and delays it to about 5 hours, without changing the total absorbed. [1], [63]

The half-life is about 50 hours on the US label and 40–50 hours on the UK label; the male study measured 41–46 hours. With daily dosing, levels climb for about 7 days to a steady state 3 to 4 times higher than after a single dose. About 85% is cleared by the liver and about 10% by the kidneys. [1], [11], [63]

Can an anastrozole tablet be split?

The labels describe a white, film-coated, round tablet that is not scored, and none gives splitting instructions or tests how evenly the pieces divide. A quarter tablet is therefore an estimate, not a measured 0.25 mg, and small errors matter more at small doses. Compounded lower-strength capsules or liquids are sometimes offered instead; compounded drugs are not FDA-approved, which means FDA does not check their safety, effectiveness or quality. [1], [10], [11], [64]

How should anastrozole be stored?

Store it at room temperature in its original pack. Each product's label sets its own details, so the leaflet in your box is the final word. [1], [11], [25]

[1], [10], [11], [65], [25]

For carrying medicines across borders, see traveling with testosterone.

What does anastrozole do to fertility, estrogen and red blood cells?

Each of these has its own hub guide. Here is what is specific to anastrozole.

Fertility. Used instead of testosterone, it raises LH and FSH and improved sperm counts in some men with low testosterone and low counts; taken alongside TRT, it does not protect sperm production (details above). See TRT and fertility. [16], [47], [24]

Estrogen. It lowers estradiol by roughly a third to two thirds in men: from 26 to 17 pg/mL in older men not on TRT, and from a median of 65 to 22 pg/mL in the on-TRT clinic series. That is its purpose and its main risk. See estrogen on TRT. [29], [16], [3], [5]

Red blood cells. Hematocrit did not change in the 12-week or 1-year trials of anastrozole instead of TRT; in 51 men on clomiphene plus anastrozole, 2 had hematocrit above 54%. Men on TRT keep the usual checks; see hematocrit on TRT. [16], [17], [32]

What do we actually know about anastrozole in men?

A fair amount about what it does to hormones, a little about fertility, and almost nothing from trials about adding it to TRT.

What did the randomized trials in men find?

The trials are small, short and mostly in older men not on TRT; the research trial with testosterone and the clomiphene comparison are described above. They agree on one thing: anastrozole moves hormone numbers reliably, and those numbers have not translated into measured benefits. [17], [16], [6]

Doserly

Anastrozole used instead of testosterone in older men

Randomized trials in older men with low or borderline testosterone who were not on TRT.

BeforeAfter

Leder 2004: 12 weeks, 37 men aged 62–74

1 mg daily: testosterone rose

343 to 572 ng/dL

1 mg daily: estradiol fell

26 to 17 pg/mL

1 mg twice a week: testosterone rose

397 to 520 ng/dL

1 mg twice a week: estradiol fell

27 to 17 pg/mL

Quality of life, erections, urinary symptoms and hematocrit did not change. PSA rose from 1.7 to 2.2 ng/mL in the twice-a-week group only.

Burnett-Bowie 2009: 1 year, 69 men aged 60+

Anastrozole 1 mg daily: spine bone density fell

1.121 to 1.102 g/cm² (−0.019)

Placebo: spine bone density rose slightly

1.180 to 1.189 g/cm² (+0.009)

Difference between groups: P = 0.0014.

Did not change (Burnett-Bowie 2009, 88 men, 1 year)

• Body composition
• Strength
• PSA
• Urinary symptoms
• Hematocrit
• Lipids

Hormone numbers moved; measured benefits did not follow, and spine bone density fell.

The on-TRT clinic series, the pellet study and the fertility series report changes without control groups, so some improvement may have happened anyway, and the clinician survey shows how often anastrozole is prescribed, not whether it helps. [3], [21], [19], [33]

What is still unknown?

• Whether adding anastrozole to TRT improves how men feel, beyond one small trial in men with epilepsy.
• What it does to bone over more than a year, and whether it raises fracture risk in men.
• Its effect on heart rhythm, heart events or mood in men on TRT: no study has isolated it.
• The right dose, if any, for men on TRT, and whether low intermittent doses or cut tablets behave predictably.

[31], [22], [15]

How does anastrozole work?

Anastrozole blocks aromatase, the enzyme that turns testosterone into estradiol and androstenedione into estrone. It binds reversibly: when the drug clears, the enzyme works again. Exemestane, a related drug, binds permanently, so the body has to make new enzyme. [4], [1]

Where men's estradiol comes from. Men make about 35–45 micrograms of estradiol a day. About 20% comes straight from the testicles, and most of the rest is made by converting testosterone and related hormones in fat tissue, muscle, brain, bone and other tissues. Aromatase is also active in fat tissue, and in one fertility practice, men with obesity and higher starting estradiol were more likely to have anastrozole added. [4], [67]

Why it raises testosterone off TRT but not on it. Estradiol acts as a brake on both the hypothalamus and the pituitary. Lowering it releases the brake: LH rises, and working testicles make more testosterone. On full-dose TRT the brake is largely on from the prescribed testosterone, so estradiol falls and, in the one clinic series, testosterone stayed where the dose put it. With waning pellets, one records review found that adding anastrozole kept testosterone up longer. [28], [16], [3], [21]

Doserly

The same tablet, off TRT and on TRT

Before and after anastrozole in two different studies. Each marker uses the same scale in both.

BeforeAfter

Off TRT: Leder 2004

Randomized trial, 12 older men, 1 mg daily for 12 weeks (means).

Estradiol fell

26 to 17 pg/mL

LH rose

5.1 to 7.9 U/L

Testosterone rose

343 to 572 ng/dL

On TRT: Punjani 2021

Clinic records, 44 men, 0.5 mg three times a week (medians).

Estradiol fell

65 to 22 pg/mL

Testosterone stayed the same

616 and 596 ng/dL

LH was measured and did not change significantly (values not published). Different men, doses and study types; not a head-to-head comparison.

Off TRT it raised testosterone; on TRT it only lowered estradiol.

How strong the effect is. In men, drugs like anastrozole lower the estradiol-to-testosterone ratio by about 77% and do not suppress estradiol completely, probably because men have far more testosterone, the raw material for estradiol, in their blood. It does not affect the adrenal hormones cortisol or aldosterone, and it does not block estrogen receptors; that is what tamoxifen does. [4], [11], [1]

Why bone is the main concern. In men, estradiol made from testosterone is what keeps bone strong. In the trial comparing gel with anastrozole, both raised testosterone, but only the gel, which also raises estradiol, improved spine bone density. Men born without working aromatase have very low bone mass. [6], [4]

What are the risks and unanswered questions?

Is anastrozole safe?

For a short, supervised course it is usually tolerated, but lowering estradiol has real costs in men: weaker bones, more body fat and lower sex drive when estradiol goes too low. Most label safety data come from women with breast cancer taking it for years, and no long-term safety study exists in men on TRT. [22], [5], [1]

What side effects do the labels list?

In the large breast cancer trial that compared anastrozole with tamoxifen in about 6,200 women (3,092 on anastrozole and 3,094 on tamoxifen), the US label reports:

[1]

Hot flashes, headache, nausea, rash and weakness are also common in women, and the UK label lists hair thinning as common and trigger finger as uncommon. Tendon problems, including tendon rupture, have been reported after marketing. The 2026 Brazilian statement lists these possible effects in men: high blood pressure, higher PSA, headaches, lower bone density, joint and muscle pain, fatigue, mood changes, raised liver enzymes, higher hematocrit and higher LDL cholesterol. [1], [11], [2]

What happens when estradiol goes too low?

What is measured in men:

• Body fat and sex drive. When researchers lowered men's hormones on purpose, missing estradiol mainly caused fat gain, and both testosterone and estradiol contributed to sexual function. [5]
• Libido. In a randomized trial of 296 infertile men, lower libido was reported by 18 of 147 on letrozole, another aromatase inhibitor, against 8 of 149 without it. [68]
• Blood sugar. Six weeks of 1 mg daily made insulin work less well in healthy men. [66]
• Memory. In older men given testosterone, verbal memory improved only when estradiol was allowed to rise. [20]

Joint pain, low mood and poor sleep are label effects in women and common complaints in men's forums, but they have not been measured in trials of men on TRT. On-TRT accounts of "crashed" estradiol are described in the community section. [1]

Does anastrozole weaken bones?

In older men it lowered spine bone density within a year, and long-term male data do not exist. In 69 men aged 60 and older, a year of 1 mg daily lowered spine bone density from 1.121 to 1.102 g/cm², while it rose from 1.180 to 1.189 g/cm² on placebo. Other sites moved the same way without reaching statistical significance. [22]

• A meta-analysis found spine bone density lower after 6 months on an aromatase inhibitor. [69]
• An investigational aromatase inhibitor lowered spine bone density by 1.2% to 2.1% over 24 weeks, against a 0.7% rise on placebo. [39]
• In the UK regulator's reading of the trial in boys with growth hormone deficiency, fractures were more common on anastrozole. [11]

The labels warn that it can lower bone density and advise considering bone density checks; the AUA says aromatase inhibitors should in general not be used for long periods because of bone loss, and the Brazilian statement rules them out with osteoporosis or marked osteopenia. No study has counted fractures in adult men. [1], [15], [2]

Does anastrozole affect the heart, cholesterol or blood sugar?

In women with existing heart disease, heart events were more common on anastrozole; in men, the short trials found no change in cholesterol, and no heart-outcome study exists. [1], [17]

• Heart events (women). In the 465 of 6,186 women who already had ischemic heart disease (heart disease caused by narrowed arteries), ischemic heart and blood-vessel events occurred in 17% of the 216 on anastrozole and 10% of the 249 on tamoxifen. [1]
• Cholesterol. In women, high cholesterol was reported in 9% on anastrozole vs 3.5% on tamoxifen (3,092 vs 3,094 women), and in two earlier trials in advanced breast cancer, average total cholesterol rose by about 0.5 mmol/L on anastrozole; a later label trial found no meaningful change in LDL or HDL over 12 months. In older men, fasting lipids did not change over 12 weeks or 1 year, and in the epilepsy trial triglycerides fell with anastrozole. [1], [70], [17], [31]
• Blood sugar. Insulin sensitivity got worse in healthy men over 6 weeks, but did not change over 12 weeks or 12 months in older men with low testosterone. [66], [70], [71]

The VA cites heart events, belly fat and bone loss as its reasons against routine use. Atrial fibrillation comes up in forums, but no study has linked it to anastrozole. [7]

For testosterone itself, the best heart evidence is TRAVERSE, a trial of about 5,200 men at high heart risk given testosterone gel or placebo gel; it did not test anastrozole. Heart attack, stroke or heart death occurred in 182 of 2,596 men on testosterone (7.0%) vs 190 of 2,602 on placebo (7.3%) (hazard ratio 0.96, a measure of relative risk), meeting its safety margin, while atrial fibrillation (3.5% vs 2.4%), acute kidney injury (2.3% vs 1.5%) and lung clots (0.9% vs 0.5%) were more common. In TRAVERSE, clinical fractures were more common on testosterone (3.50% vs 2.46%; 91 of 2,601 vs 64 of 2,603 men; hazard ratio 1.43), so testosterone did not protect bone. It tested only a gel, men used it for 21.7 months on average and about 61% stopped early. See TRT and heart health. [72], [73], [74]

Could anastrozole affect the prostate?

In the 12-week trial, PSA (prostate-specific antigen, a prostate blood test) rose from 1.7 to 2.2 ng/mL in the twice-weekly group only; in the 1-year trial, PSA and urinary symptoms did not change. The Brazilian statement lists a PSA rise among possible effects. Prostate checks on TRT continue as usual; see TRT and the prostate. [16], [17], [2]

Is anastrozole legal? Is it FDA approved?

Checked September 2026. Rules differ by country and change often.

• United States. Anastrozole is FDA-approved as a prescription medicine for breast cancer in women after menopause, first as Arimidex on December 27, 1995; the brand is now sold by ANI Pharmaceuticals, and generics are marketed. It is not a controlled substance: DailyMed lists no DEA schedule, and it is absent from the DEA's controlled substances list dated August 27, 2026. Every use in men is off-label. FDA's drug shortage data list no anastrozole shortage (updated September 26, 2026). [8], [9], [75], [15], [76]
• Canada. Prescription-only (on the Prescription Drug List). On September 28, 2026, Health Canada's Drug Product Database listed 18 anastrozole products as marketed: Arimidex and 17 generics. The monograph covers breast cancer in women after menopause only. [77], [12], [25]
• United Kingdom. A prescription-only medicine, licensed for breast cancer treatment and prevention in women after menopause. [11]
• Australia. Prescription-only (Schedule 4). The Pharmaceutical Benefits Scheme subsidizes it only for breast cancer, so a man's prescription is not subsidized. [13], [14]
• European Union. Authorized country by country; a 2011 European review harmonized the Arimidex product information, for breast cancer in women after menopause only. [26]

Buying anastrozole labeled "for research use only" does not make it legal, tested or the same as a licensed medicine.

What it costs. It is inexpensive as a generic. In September 2026 US pharmacies paid about $0.14 per generic tablet and about $52.86 per Arimidex tablet (the average acquisition cost, not the retail price). The NHS reimbursement price in England was 105 pence for 28 tablets, and the Australian subsidized price for breast cancer was A$20.77 for 30 tablets. [78], [79], [14]

Is anastrozole banned in sport?

Yes. Aromatase inhibitors, anastrozole included, are in class S4.1 of the World Anti-Doping Agency (WADA) Prohibited List, banned in and out of competition, and the 2027 list keeps them banned from January 1, 2027. [80], [81]

Who should be especially cautious?

• Men with osteoporosis or thin bones, or at high fracture risk. The Brazilian statement rules it out with osteoporosis or marked osteopenia, and bone density fell in older men. [2], [22]
• Men with severe liver disease. A contraindication in the Brazilian statement; the UK label advises caution with moderate to severe liver impairment. [2], [11]
• Men with normal or low estradiol, or no symptoms, since the measured harms in men come from lowering it (low estradiol). [5]
• Men with existing heart disease, diabetes or insulin resistance. Heart events were more common in women with heart disease, and insulin sensitivity fell in healthy men. [1], [66]
• Men who want children while on TRT. Anastrozole does not protect sperm production on testosterone (fertility). [24]
• Men taking tamoxifen or estrogen-containing medicines (see the next question).
• Children and teenagers. The UK label says it is not recommended for children and adolescents; any use is specialist care. [11]
• Drug-tested athletes. Banned at all times. [80]
• Women who are pregnant or breastfeeding. It is contraindicated and can harm an unborn baby; these guides are for men. [11], [1]

Can anastrozole be taken with other medicines?

The labels list few interactions:

• Tamoxifen: do not combine. Taking both lowered anastrozole levels by 27% in a large trial. The UK anastrozole label says to avoid the combination, and the US tamoxifen label says the two should not be given together. [23], [11], [82]
• Estrogen-containing medicines may weaken its effect and should not be used with it. [1]
• Warfarin. In 16 male volunteers, anastrozole did not change warfarin levels or clotting. [1]

hCG. hCG makes the testicles produce testosterone and some estradiol. One review reports that letrozole worked almost as well (81% vs 77% lower estradiol-to-testosterone ratio) when the testicles' own hormone output was suppressed, so the testicles do not seem to explain the incomplete effect; no trial found for this guide tested anastrozole together with hCG. See hCG. [4]

Supplements. Evidence that supplements lower estradiol in men is very thin. One small study of 8 healthy men, with no separate control group, found estradiol fell after a week of 10 mg of boron a day. No trial found for this guide tested boron, DIM, calcium D-glucarate or zinc in men on TRT, so none can be called a substitute. [83] See the DIM, calcium D-glucarate, zinc and boron guides for what is known about each. Bring a full list of everything you take to a pharmacist or prescriber.

What should be monitored while taking anastrozole?

The US label advises considering bone density and cholesterol monitoring. The AUA adds estradiol and testosterone checks when anastrozole is used instead of testosterone. No guideline sets a testing schedule for men taking it with TRT. Use this summary to prepare questions for your prescriber; it is not a personal testing plan. [1], [15]

[1], [15], [2], [60]

See TRT blood work. When anastrozole starts, stops or changes, note the date, and record when the last tablet and last testosterone dose were taken before each draw. [1]

What do people in public communities report?

Public forums hold every kind of anastrozole story: men who felt better within a day, men who felt worse within a day, and many more asking whether to take a prescription at all. The accounts below come from public Reddit threads posted between September 2025 and September 2026, described without identifying anyone. Threads about non-prescribed steroid cycles appear only where they show something about anastrozole itself, and are marked.

Why are men offered anastrozole?

In most threads, the prescription followed a lab number rather than a symptom. One man felt good after starting TRT, with estradiol at 50; his doctor raised his testosterone to 140 mg a week and added 1 mg of anastrozole weekly anyway. After the second dose his sex drive and erections worsened, his estradiol tested below 5, and he skipped the next dose. [84]

Others paused. A man who felt much better after losing 100 pounds on TRT was prescribed 1 mg every 48 hours for an estradiol of 59.7 with no symptoms, and planned to ask about repeating the labs. Another, offered 0.5 mg for an estradiol of 40 pg/mL a month into weekly injections, chose instead to split his dose into two injections and retest. [85], [86]

What do "crashed estradiol" stories look like?

This is the most common harm described, and the numbers in several accounts back it up.

• Weekly 1 mg for a mild result. A man who felt fairly good on 100 mg a week was given 1 mg weekly for an estradiol of 43 without symptoms. His energy and libido dropped after the first dose; later, estradiol was below the lab's measurable limit, with aches, poor sleep and irritability. His hematocrit had also reached 60%, which he blamed on anastrozole and a reply disputed. [87]
• Daily 1 mg with pellets. After a little over a month of 1 mg daily, a man on pellets developed fatigue and anxiety, and his estradiol came back below the reportable range. [88]
• Five doses were enough. Another man took 0.5 mg twice a week for an estradiol of about 60 with no clear symptoms. Within about two weeks he described severe fatigue, poor sleep and a loss of desire and erections, stopped the tablets and lowered his testosterone. [89]

One man put the opposite side plainly: after five months on 200 mg a week with 1 mg of anastrozole weekly from the start, he blamed joint pain, erection problems and low mood on the tablet, stopped it with his prescriber's agreement and felt much better. Three weeks later his estradiol was 66, and the prescriber asked him to restart based on the number. [90]

What do the positive reports look like?

Some men describe clear relief, usually when there were symptoms to begin with:

• A man on 120 mg a week with bloating, nipple sensitivity and sexual problems said these eased quickly on 0.5 mg a week, then planned to split both his testosterone and his anastrozole into twice-weekly doses and retest before trying to stop it. [91]
• A man whose sleep and anxiety worsened after adding hCG, with estradiol at 85 pg/mL, described much deeper sleep within two days of anastrozole. He had not rechecked his estradiol when he posted. [92]
• Itchy nipples at an estradiol of 55 pg/mL mostly settled for one man taking 0.5 mg every two to three days, less than his doctor had prescribed. [93]

None of these men reported a follow-up over months.

Why is it so hard to tell what helped?

Because two things usually change at once. A man with chest acne on 200 mg a week lowered his dose to 160 mg and started an aromatase inhibitor in the same week. The acne improved, but his libido and morning erections disappeared, and he could not tell which change was responsible; he planned to stop the tablet and keep the lower dose. [94]

Another man felt his best on 60 mg a week; when his doctor raised it to 100 mg and added 0.5 mg of anastrozole weekly, the benefits faded, and two weeks after stopping the tablet he could not tell whether it, the higher dose or simply time was to blame. [95]

A single dose can look like proof in either direction. One man on 125 mg a week felt better sleep and erections for one night after 0.25 mg, then the problems returned. [96]

What do people say about finding a dose?

Dose-finding runs into the 1 mg tablet:

• Cutting tablets. One man found it hard to divide unscored 1 mg tablets into consistent quarters and bought a second pill cutter. [91]
• Very small doses. Another, who had felt awful when estradiol went too low before, tried 0.03125 mg (a thirty-second of a tablet) on one night a week. [97]
• Too much, then too little. A man with a big estradiol rise felt low-estradiol symptoms on 0.25 mg three times a week and high-estradiol symptoms on 0.125 mg twice a week, and was still searching. [98]
• Unit mistakes. One man first wrote his dose as 6.25 mg, then 25 mg, before correcting it to 0.25 mg twice a week. [99]

Timing confuses results too. One man's estradiol of 28 was drawn two days after a dose of anastrozole, so it could not show his usual level. [100]

Which other problems do people blame on anastrozole?

Several threads blame anastrozole for problems the thread cannot settle:

• Heart rhythm. A man on 0.5 mg twice a week developed atrial fibrillation for the first time after a stressful week; his cardiologist attributed it to stress. [101]
• Lasting sexual symptoms. One man described the same episode across three threads: genital numbness and sexual dysfunction starting after a single 0.25 mg dose, taken hours after a testosterone-booster supplement and during a two-week break from his TRT. His later estradiol results were not low, and replies questioned the explanation. [102], [103], [104]
• Hair shedding. A man took 0.25 mg every two days for 15 days and then had heavy shedding for over a month. His weekly estradiol results fell only from 85 to 65 pg/mL, which he pointed out was not a crash. [105]
• Cholesterol. A man on testosterone, hCG and anastrozole noted HDL (the "good" cholesterol) of 39.7 mg/dL, against about 55–57 before treatment, and wondered which drug was responsible. [106]
• A bad reaction to a first dose. After one 0.5 mg dose for nipple sensitivity, one man described hot flashes, flushing, palpitations and higher blood pressure for a couple of days and did not take a second dose. [107]

These are single accounts. None shows that anastrozole caused the problem, and none shows that it did not.

What about anastrozole without TRT?

Some men take it with clomiphene or enclomiphene instead of testosterone. One described less brain fog and better erections after three months, while starting weight training at the same time. Another, trying for a child, stopped 1 mg twice a week because of joint pain, felt very well on 0.25 mg twice a week, and asked whether an estradiol of 90 meant he should take more; replies suggested judging by symptoms. [108], [109]

Threads about steroid cycles, with doses far above replacement, are outside this guide (TRT vs steroids). One shows the drug's limits: a man on a 460 mg weekly cycle added anastrozole for a tender breast lump, which felt slightly larger two days later. (Steroid cycle.) [110]

How can you judge an anastrozole story?

Ask: Were there symptoms, or only a lab number? Which estradiol test was used, and how long after the last testosterone injection and the last tablet was blood drawn? What was the dose in milligrams, and how was the tablet cut? What else changed at the same time, such as the testosterone dose, hCG, weight loss or training? And what happened a few months later?

These selected discussions show what people experience and the questions research has not answered. They are not a survey of all users, a success rate or a substitute for the studies.

How does anastrozole compare with other options?

These are the options most often weighed against anastrozole by men on TRT. Status was checked in September 2026.

No trial has compared these options in men on TRT. See estrogen on TRT for the full picture.

Common questions about anastrozole

Do I need an aromatase inhibitor on TRT?

Usually not. Most men on TRT would not qualify under the guidelines that address it: the VA does not generally recommend aromatase inhibitors, the European urology guideline calls the evidence poor, and the AUA reserves them for persistent breast symptoms with high estradiol. Under one clinic's published criteria, only 44 of 1,708 men qualified. [7], [41], [15], [3] See should it be added.

What dose of anastrozole is used on TRT?

No label or major guideline gives one. The only published on-TRT clinic schedule is 0.5 mg three times a week, adjusted to keep estradiol at 20–40 pg/mL, and a clinician survey found starting doses from 1 mg a week to 1 mg a day. The guideline ranges (AUA 0.05–1 mg every 1–3 days as printed; Brazil 0.5–1 mg daily or every other day) are for use instead of testosterone. [3], [33], [15], [2] See typical protocols.

Should I target a specific estradiol number?

No guideline sets an estradiol target for men on TRT. The AUA acts on breast symptoms with high estradiol and suggests referral if estradiol is persistently above 40 pg/mL before treatment; the Brazilian statement advises against routine estradiol checks on TRT. Standard immunoassays can also read men's estradiol too high. [15], [2], [60] See which level.

What happens if anastrozole lowers estradiol too far?

In men, low estradiol is linked to fat gain, lower sex drive and poorer insulin sensitivity, and over time weaker bones. When it stops, the drug clears within about a week (its half-life is about 2 days); in women, estradiol stayed suppressed for up to 6 days after stopping. Report symptoms to your prescriber rather than adjusting doses on your own. [5], [66], [22], [1] See low estradiol.

Is anastrozole safe to take long term?

Its long-term safety in men has not been studied. A year of 1 mg daily lowered spine bone density in older men, and the AUA says aromatase inhibitors should in general not be used for long periods. The labels advise considering bone density and cholesterol checks. [22], [15], [1] See bone.

Can I use anastrozole instead of TRT?

It can raise testosterone in men whose testicles and pituitary work, and some guidelines list it as an off-label option for men who want to keep fertility. But in the largest year-long placebo-controlled trial, body composition and strength did not change and spine bone density fell; a smaller trial saw lean mass rise about 1.2–1.5 kg from baseline, while only testosterone gel improved bone and walking speed. Clomiphene raised testosterone more in a head-to-head trial. [16], [17], [15], [18] See instead of TRT.

Can I cut a 1 mg tablet into quarters?

The labels describe an unscored, film-coated tablet and give no splitting instructions or data on how evenly it divides, so a quarter is an estimate rather than a measured 0.25 mg. Compounded lower-strength versions are not FDA-approved. Ask your prescriber or pharmacist before changing how you take it. [10], [11], [64] See tablets.

Is anastrozole a steroid, a controlled drug or banned in sport?

It is not a steroid, and it is not a controlled substance in the US; everywhere covered here it is a prescription-only medicine. It is banned in sport at all times (WADA class S4.1). [1], [9], [13], [80] See legal status.

Glossary

Plain explanations of the medical, dosing and research terms used in this guide. Underlined terms in the text link here.

Aromatase

The enzyme that turns testosterone into estradiol (and androstenedione into estrone). It is found in fat tissue, the testicles, the brain and bone.

Aromatase inhibitor

A medicine, such as anastrozole, letrozole or exemestane, that blocks the enzyme that turns testosterone into estradiol, so estradiol falls.

Azoospermia

No sperm at all in a semen sample.

Bone mineral density

A measure of how much mineral is packed into bone, usually taken with a low-dose X-ray scan (DXA) of the spine and hip. Lower values mean weaker bone.

Controlled trial

A study that compares people who receive a treatment with a similar group who do not, often receiving a placebo instead. Randomly assigning people to each group makes the comparison fairer.

Dose (amount each time)

The amount given on one occasion. A protocol lists both the dose and how often it is given.

Estradiol

The main form of estrogen. In men it is made mostly from testosterone and helps keep bones strong, control body fat and support sex drive.

FSH (follicle-stimulating hormone)

A pituitary hormone that supports sperm production. Anastrozole raises it in men not on TRT; testosterone therapy lowers it.

GnRH agonist

A medicine that, given continuously, shuts down the pituitary's LH and FSH signals. It is paired with an aromatase inhibitor in male breast cancer.

Gynecomastia

Growth of breast tissue in men, often tender at first. It is linked to a higher estrogen-to-testosterone balance.

Half-life

The time it takes for the level of a substance in the blood to fall by half. For anastrozole it is about 50 hours. It is not the same as how long an effect lasts.

Hematocrit

The share of the blood made up of red blood cells. Testosterone therapy can raise it, which is why it is checked.

Immunoassay

A common, inexpensive lab method that measures a hormone using antibodies. For the low estradiol levels in men, direct immunoassays can read too high.

Mass spectrometry (LC-MS/MS)

A more accurate lab method that measures a hormone by its molecular weight. It is often labeled a "sensitive" estradiol test.

LH (luteinizing hormone)

A pituitary hormone that tells the testicles to make testosterone. Anastrozole raises it in men not on TRT; testosterone therapy switches it off.

Meta-analysis and systematic review

A systematic review gathers all studies on a question in a planned way. A meta-analysis combines their results statistically.

Off-label use

Prescribing an approved medicine for a use, dose or group of people that its label does not cover. It is legal for prescribers but means the regulator has not reviewed evidence for that use.

Osteopenia and osteoporosis

Osteopenia is bone density below normal; osteoporosis is thinner, weaker bone with a higher chance of breaking.

Post-cycle therapy (PCT)

Drugs taken after an anabolic-steroid cycle to try to speed the return of the body's own testosterone. The schedules used online have not been tested in trials.

Placebo

A dummy treatment with no active ingredient, used as a comparison in studies.

Primary hypogonadism

Low testosterone because the testicles themselves do not work well, even though the pituitary sends plenty of LH and FSH. Anastrozole cannot raise testosterone here.

PSA (prostate-specific antigen)

A blood test for a protein made by the prostate, used to decide whether prostate checks are needed.

SERM (selective estrogen receptor modulator)

A tablet, such as tamoxifen, clomiphene or enclomiphene, that blocks estrogen's action in some tissues rather than lowering estrogen. At the brain it blocks estrogen's feedback, so the pituitary releases more LH and FSH.

Steady state

The point, after repeated doses, when the amount taken in matches the amount cleared, so blood levels stop climbing. For daily anastrozole it takes about 7 days.

Testosterone-to-estradiol ratio

Total testosterone (in ng/dL) divided by estradiol (in pg/mL). Some fertility studies selected men with a ratio below 10; no best value has been established.

Titration

Adjusting a dose in steps, usually based on blood tests and symptoms.

TRT (testosterone replacement therapy)

Prescribed testosterone, by injection, gel or other forms, for men with low testosterone. It switches off the brain's LH and FSH signals.

WADA

The World Anti-Doping Agency, which publishes the list of substances banned in sport. Aromatase inhibitors are banned at all times.

Weekly total

All the amounts taken in one week, added together. For example, 0.5 mg three times a week is a 1.5 mg weekly total.

How this guide was researched

This guide is built from a thorough review of the sources cited throughout it: product labels from the United States, the United Kingdom and Canada, clinical guidelines from the United States, Canada, the United Kingdom, Europe and Brazil, published clinical trials and studies, and regulatory records from four countries. We also reviewed public online forums where men describe their own experiences with anastrozole.

The guide cites 115 sources, including 10 product labels, 15 clinical guidelines and position statements and 38 original studies in people, plus 27 public community threads. Each type of source answers a different question. Labels and guidelines show what is approved and what clinicians are told to do. Trials and studies show what was measured, and in whom. Personal accounts show what individual people experienced. Every numbered citation links to its entry below, labeled by source type.

How this guide was made

Research and drafting were AI-assisted. Every cited source was checked against the original, and the guide was reviewed and edited by Doserly before publication. It has not had an independent clinical review, and Doserly does not currently have medical reviewers. Doserly makes a medication and health-tracking app and runs Doserly Academy, both of which are promoted in this guide. Read our editorial policy for how guides are researched, updated and corrected.

This guide is for educational purposes. It summarizes what the reviewed sources report so the research is easier to understand; it is not medical advice. For a deeper dive, or to check any point for yourself, go straight to the cited sources.

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