Aromatase inhibitors in men

Anastrozole, letrozole and exemestane are licensed for breast cancer in women. Everything below is off-label use in men, and the evidence base is much thinner than the confidence with which these drugs are discussed.

What the evidence shows

Findings are grouped by what kind of evidence each one is. That ordering is the point: a sentence from an FDA label and a sentence from a forum are not the same claim, and this page will not present them as though they were.

Regulator-reviewed (FDA label)

• The anastrozole label's own bone warning documents that the licensed 1 mg/day dose lowers bone mineral density at both the spine and hip. This is the regulator-reviewed statement of the drug's skeletal effect at the only dose the FDA has ever evaluated, and it is a decrease, not a neutral effect.

  Dose or parameter studied: 1 mg once daily (the only FDA-evaluated dose, in postmenopausal women)

  Studied in: Postmenopausal women with early breast cancer in the ATAC trial bone substudy. No male data of any kind appears in the label. Men on testosterone are not in the studied population, and men have a different baseline estradiol source (peripheral aromatization of testosterone rather than ovarian output).

• The letrozole label quantifies the bone cost of full-dose aromatase inhibition: a median 4.1% loss of lumbar spine BMD over 24 months versus a 0.3% gain on tamoxifen. In the extended adjuvant setting hip BMD fell a median 3.8% at 2 years versus 2.0% on placebo.

  Dose or parameter studied: 2.5 mg once daily

  Studied in: Postmenopausal women in the adjuvant and extended adjuvant letrozole BMD substudies (MA-17B). Not men, and not men receiving exogenous testosterone. Cited here to establish the magnitude of BMD loss achievable with sustained, full aromatase blockade in humans.

Clinical trial evidence

• In a 2,639-man prospective cohort with mass-spectrometry hormone measurement, the relationship between estradiol and fracture was non-linear with a sharp inflection below 16 pg/mL. Free estradiol, not free testosterone, was independently associated with fracture risk after adjustment. [12]

  Dose or parameter studied: Serum estradiol below ~16 pg/mL marks the inflection for rising fracture risk (observational threshold, not a dose)

  Studied in: 2,639 community-dwelling Swedish men, mean age 75, followed a mean 3.3 years with validated fractures (MrOS Sweden). Observational — these were men with naturally low estradiol, not men whose estradiol was pharmacologically suppressed, and no causal inference about AI use follows. But it is the only human fracture-outcome data anywhere near this question.

• In the US MrOS cohort, men in the lowest bioavailable estradiol group (<11.4 pg/mL) had a 1.5-fold higher risk of non-vertebral fracture — while men in the lowest bioavailable testosterone group had NO increased fracture risk once estradiol was adjusted for. For male bone, the estradiol number carries the risk signal and the testosterone number does not. [13]

  Dose or parameter studied: Bioavailable estradiol below 11.4 pg/mL (observational risk threshold, not a dose)

  Studied in: Case-cohort within 5,995 community-dwelling US men aged 65+; subcohort of 1,436 white men plus all 446 minorities and all incident hip and non-vertebral fractures. Testosterone and estradiol by mass spectrometry. Observational and in elderly men; does not establish that lowering estradiol with a drug causes fractures, but it is the reason the bone concern is taken seriously.

• The natural experiment of congenital aromatase deficiency shows what a permanently crashed estradiol does to a male skeleton: every reported man has osteopenia or osteoporosis and unfused epiphyses, despite normal or high testosterone. Replacement is with estradiol — about 25 micrograms transdermally per day — and testosterone does not substitute for it. [14]

  Dose or parameter studied: Transdermal estradiol approximately 25 micrograms daily for lifelong replacement in aromatase-deficient men

  Studied in: Men with congenital loss-of-function CYP19A1 mutations — a handful of case reports worldwide. This is lifelong, complete estrogen absence from before puberty, not the partial, reversible suppression a man gets from an AI. It establishes the direction and biological importance of the effect, not its magnitude at any particular AI dose.

• The testosterone-to-estradiol ratio that circulates widely as a target has no validated optimum. A 2024 review found the data on concurrent estradiol values in men on testosterone therapy or aromatase inhibitors to be scant, and concluded the optimal ratio has not been determined — while noting that higher ratios track with REDUCED bone density. [15]

  Studied in: Narrative review of reported T:E2 ratios in men, both natural and after pharmacologic manipulation. Not a trial. The review explicitly flags assay separation, measurement technique, sample timing and lack of normative data as unresolved limitations — meaning two labs can hand a man two different ratios from the same blood.

Off-label use of an approved drug

• The only randomised design that isolates the effect of adding an aromatase inhibitor on top of a fixed exogenous testosterone dose found that estrogen deficiency — not androgen deficiency — was what drove fat gain, and that both testosterone and estradiol contributed to sexual function. Suppressing estradiol while testosterone is held constant makes men fatter and worsens sexual function. [1]

  Dose or parameter studied: Anastrozole 1 mg daily added to goserelin plus testosterone gel at 0, 1.25, 2.5, 5 or 10 g/day for 16 weeks

  Studied in: 202 healthy men aged 20-50 with normal baseline testosterone, chemically castrated with goserelin and then given graded testosterone gel plus anastrozole (compared against 198 men given the same testosterone doses without anastrozole). This is the closest published analogue to a man on testosterone who adds an AI — but the testosterone doses were replacement-range, the men were young and healthy, and the exposure was only 16 weeks.

• In the same trial's bone analysis, spinal BMD by quantitative CT fell substantially in every testosterone dose group in which aromatization was blocked — and the fall was independent of testosterone dose. Taking more testosterone does not protect the spine if the estradiol is being suppressed. Estradiol deficiency also degraded cortical bone microarchitecture at peripheral sites. [2]

  Dose or parameter studied: Anastrozole 1 mg daily on top of testosterone gel 0-10 g/day for 16 weeks

  Studied in: 202 healthy men aged 20-50 on goserelin plus graded testosterone plus anastrozole; bone microarchitecture assessed in 100 men. Sixteen weeks only. Fracture was never an endpoint. Men self-managing testosterone typically use AIs far longer than 16 weeks, and often at supraphysiologic testosterone — neither condition was tested.

• The single most-cited estradiol number in this space — E2 above roughly 10 pg/mL — comes from this trial. It is a FLOOR below which bone resorption rises and BMD falls, not a validated therapeutic target and not an optimum. Testosterone above about 200 ng/dL was the parallel androgen floor. [2]

  Dose or parameter studied: Estradiol >10 pg/mL and testosterone >200 ng/dL (thresholds, not doses)

  Studied in: Healthy men aged 20-50 in a 16-week GnRH-agonist clamp with graded testosterone with or without anastrozole. The thresholds were derived by grouping men by achieved hormone level within that experiment. They were never prospectively tested as treatment targets, never validated against fracture, and never derived in men at steady state on exogenous testosterone outside a research clamp.

• A full year of anastrozole 1 mg daily in older men raised testosterone and lowered estradiol from 15 to 12 pg/mL — a modest drop — and spine BMD still fell significantly compared with placebo. The authors' conclusion is that aromatase inhibition does not improve skeletal health in these men. [3]

  Dose or parameter studied: Anastrozole 1 mg daily for 12 months

  Studied in: 69 men aged 60+ with borderline or low testosterone and hypogonadal symptoms, NOT on exogenous testosterone. This is AI monotherapy in men with an intact HPG axis — a different question from adding an AI to injected testosterone. Note that the harm appeared at an estradiol level (12 pg/mL) that many men self-managing would consider acceptable or even high.

• In a separate 1-year placebo-controlled trial, anastrozole 1 mg daily normalised testosterone in older hypogonadal men but produced no improvement in body composition or strength. Raising the testosterone number did not deliver the physical outcomes the number is used as a proxy for. [4]

  Dose or parameter studied: Anastrozole 1 mg daily for 12 months

  Studied in: 88 men aged 60 and older with testosterone 5.2-12.1 nmol/L and hypogonadal symptoms, on AI monotherapy without exogenous testosterone. Body composition measured by CT and DXA. Older men, replacement-range hormone shifts, no resistance training protocol — not a performance population.

• The only published head-to-head of two anastrozole regimens in men compared 1 mg daily against 1 mg twice weekly. Both raised testosterone into the youthful normal range and both lowered estradiol from about 26 to 17 pg/mL. Quality of life, erectile function (IIEF) and prostate symptom scores did not change on either regimen. The testosterone rise was driven by an LH increase — a mechanism that requires an intact HPG axis and therefore cannot operate in a man whose axis is already shut down by exogenous testosterone. [5]

  Dose or parameter studied: Anastrozole 1 mg daily versus anastrozole 1 mg twice weekly, 12 weeks (the only comparative dosing data in men)

  Studied in: 37 men aged 62-74 with screening testosterone <350 ng/dL, on AI monotherapy. Elderly, hypogonadal, not on testosterone. The twice-weekly arm is the closest published anchor for the low-dose intermittent regimens used in the community, but it was studied for 12 weeks in elderly men with no exogenous testosterone and measured no bone endpoint.

• Bone resorption starts rising within weeks of aromatase inhibition even in men whose testosterone is going UP and whose estradiol falls only modestly. Anastrozole 2 mg/day in eugonadal older men cut estradiol 29% and raised testosterone 56%, yet the resorption marker CTX climbed 11%, 24% and 33% at 3, 6 and 9 weeks. [6]

  Dose or parameter studied: Anastrozole 2.0 mg/day for 9 weeks

  Studied in: 15 eugonadal men over 65 — normal testosterone at baseline, no exogenous testosterone, no hypogonadism. Small, uncontrolled, 9 weeks. Relevant because it shows the bone signal appears fast and appears despite a rising testosterone, in men who were not hormone-deficient to begin with.

• Head-to-head over 12 months, transdermal testosterone improved lumbar spine BMD while anastrozole — which raised testosterone to comparable levels, above 500 ng/dL — did not. The authors conclude that aromatization of testosterone is required to maintain BMD in men. [7]

  Dose or parameter studied: Anastrozole 1 mg daily versus testosterone gel 5 g daily, 12 months

  Studied in: 43 men aged 65-82 with total testosterone <350 ng/dL, randomised to testosterone gel (n=16), anastrozole (n=14), or placebo (n=13). Explicitly a proof-of-concept trial; tiny arms; older men not on exogenous testosterone. The comparison is AI-instead-of-testosterone, not AI-added-to-testosterone.

• The only randomised trial that has ever added an aromatase inhibitor to injected testosterone in men enrolled 40 men for 3 months. It did not show a statistically significant sexual-function benefit for adding anastrozole, and lower estradiol correlated with better sexual function scores in this specific population — men with epilepsy, in whom relative estradiol elevation is characteristic. No bone outcome was measured. [8]

  Dose or parameter studied: Anastrozole 1 mg daily added to depot testosterone, 3 months (versus depot testosterone plus placebo)

  Studied in: 40 men with focal epilepsy, hyposexuality and hypogonadism on replacement-dose depot testosterone. This is the entire randomised evidence base for AI-on-top-of-exogenous-testosterone in men. It is 3 months, n=40, underpowered for its own primary endpoint, and in a neurological population with an atypical hormonal profile. It cannot be generalised to a healthy man on testosterone, let alone one on supraphysiologic doses.

• The only controlled letrozole dosing data in men used a starting dose of 2.5 mg per WEEK — one-seventh of the daily breast-cancer dose — titrated upward monthly. It halved estradiol and roughly 2.5-fold raised testosterone over 6 months, and produced no measurable somatic or psychological benefit on any prespecified endpoint. [9]

  Dose or parameter studied: Letrozole 2.5 mg once weekly, escalated monthly to a testosterone target of 20 nmol/L, for 6 months

  Studied in: 42 obese men (BMI >35) with total testosterone <10 nmol/L and obesity-related hypogonadotropic hypotestosteronemia; 39 completed. AI monotherapy, no exogenous testosterone. Establishes that weekly-dose letrozole is pharmacologically active in men — but the trial found no clinical benefit, and it enrolled a specific obese phenotype.

• Hot flashes and vasomotor symptoms in men are driven by estradiol deficiency, not testosterone deficiency, and the incidence steps up sharply below about 10 pg/mL — 38% of visits at E2 5-9.9 pg/mL versus 16% at 10-14.9 pg/mL. This is the clearest characterised symptom of a crashed E2 in men and it maps onto the same 10 pg/mL inflection as the bone data. [10]

  Dose or parameter studied: Anastrozole 1 mg daily added to graded testosterone gel; symptom inflection at serum estradiol below 10 pg/mL

  Studied in: 202 healthy men aged 20-50 given goserelin, graded testosterone and anastrozole, versus 198 given the same without anastrozole, plus 37 controls. Preplanned secondary analysis of the Finkelstein cohort. 16 weeks; young healthy men; symptom incidence measured per study visit, not per day.

• Exemestane's entire human evidence base in males is one 10-day crossover study in 12 subjects. It showed 25 mg and 50 mg daily suppress estradiol comparably (38% and 32%) — meaning doubling the dose bought nothing — with a reciprocal testosterone rise of about 60%, and a terminal half-life of 8.9 hours. The authors state long-term efficacy and safety still need study. [11]

  Dose or parameter studied: Exemestane 25 mg or 50 mg daily for 10 days (the only dose data in males; 25 mg gave the same estradiol suppression as 50 mg)

  Studied in: 12 healthy eugonadal males aged 14-26, normal testosterone at baseline, no exogenous testosterone, 10 days of exposure. There is no efficacy trial, no safety trial, and no bone data for exemestane in men at any duration. Anyone using exemestane in this context is operating with essentially no clinical evidence.

What the regulator approved

Where a drug on this page has a US label, this is what it was approved for and at what dose. Anything else is off-label — which is not the same as unsafe, but does mean no regulator has reviewed it for that use.

What nobody knows

These are the questions the literature does not answer. They are listed because on this subject the gaps are load-bearing: most of what circulates as settled practice sits in one of them.

• No trial has ever tested an aromatase inhibitor added to SUPRAPHYSIOLOGIC testosterone, or in any performance or bodybuilding population. The entire randomised evidence for AI-on-top-of-exogenous-testosterone is Herzog 2010 (PMID 20096638): 40 men with epilepsy, replacement-dose depot testosterone, 3 months, no bone endpoint, and the primary sexual-function difference was not statistically significant. Everything else in men is AI monotherapy.
• AI monotherapy and AI-plus-testosterone are mechanistically different questions and almost all the evidence is the wrong one. Monotherapy raises testosterone by disinhibiting LH (Leder 2004 showed LH rising from ~5 to ~8 U/L), which requires an intact HPG axis. In a man whose axis is suppressed by exogenous testosterone, that pathway is unavailable — the AI can only lower estradiol, with no compensating testosterone rise. Trials showing 'AI raises testosterone and is well tolerated' do not describe what happens to a man on TRT or a cycle.
• No estradiol target in men has ever been prospectively validated against a clinical outcome. The E2 >10 pg/mL figure (Finkelstein 2016) is a floor below which bone resorption rises inside a 16-week GnRH-agonist clamp in men aged 20-50. It was not derived as a target, not tested as a target, and not derived in men at steady state on exogenous testosterone. Nobody knows the optimal estradiol for a man on testosterone.
• Assay comparability is unresolved and matters enormously at these levels. The thresholds that exist (10 pg/mL, bioE2 11.4 pg/mL, 16 pg/mL) come from mass-spectrometry and research assays — MrOS Sweden used gas chromatography-mass spectrometry, MrOS US used mass spectrometry. Whether a routine clinical estradiol result in a man is numerically comparable to those figures has not been established, so a lab value cannot be assumed to sit on the same scale as the trial thresholds.
• There is no established dose of any AI for men. Regimens with any human data in men span letrozole 2.5 mg/WEEK (Loves 2013) to anastrozole 2 mg/DAY (Taxel 2001) — a spread of well over an order of magnitude in aromatase blockade — with no titration study against any clinical outcome. The sub-milligram, twice-weekly and 'as-needed' anastrozole regimens common in the community have no trial behind them at all; the closest anchor is anastrozole 1 mg twice weekly for 12 weeks in 11 elderly men.
• The longest AI exposure ever studied in men is 12 months (Burnett-Bowie 2009; Dias 2016), in cohorts of 43-88 men. Men self-managing hormones may take an AI for years. There is no bone, cardiovascular, cognitive or fracture data in men beyond one year, at any dose.
• Fracture has never been an endpoint in any AI trial in men. Every skeletal finding in men is BMD or a bone turnover marker — both surrogates. The only human fracture data linking low estradiol to fractures in men is observational (MrOS Sweden, MrOS US) and describes men with naturally low estradiol, not pharmacologically suppressed estradiol.
• Whether AI-induced bone loss in men reverses after stopping is unknown. No trial in men measured BMD recovery after discontinuation, so the question of whether a year of AI use leaves a permanent deficit has no answer.
• Exemestane in men rests on a single 10-day crossover in 12 healthy young males (PMID 14671195). No efficacy trial, no safety trial, no bone data, no data at any clinically relevant duration. Notably that study found 50 mg suppressed estradiol no more than 25 mg — dose-response in men is essentially uncharacterised.
• The cardiovascular direction is genuinely unresolved, not merely unstudied. Low estradiol tracks with worse bone and fracture outcomes in men, but higher endogenous estradiol independently predicted faster carotid intima-media thickness progression in middle-aged men (Tivesten 2006, PMID 16940451). Whether pharmacologically lowering estradiol in a man on testosterone helps or harms cardiovascular risk has never been tested. Anastrozole trials in older men found no improvement in lipids, CRP or insulin sensitivity (Dias 2017, PMID 27792869).
• Gynecomastia — the reason most men in this population reach for an AI — has never been the primary endpoint of a controlled AI trial in men on testosterone. Prevention of gynecomastia and treatment of established gynecomastia are different questions, and neither has been answered for this population. No dose, no duration, no expected response rate can be sourced.
• The aromatase inhibitor musculoskeletal syndrome — arthralgia, myalgia and bone pain affecting 20-47% of treated patients and often causing discontinuation — is characterised only in postmenopausal women on breast-cancer doses (PMID 33530456). Its incidence in men, at any dose, has never been measured. Men attributing joint pain to training rather than to their AI have no dataset to check against.
• Nothing is known about AIs used alongside non-testosterone anabolic compounds. Nandrolone, trenbolone and 17-alpha-alkylated orals aromatize poorly or not at all, so adding an AI to them suppresses estradiol with no aromatizable substrate to offset it — plausibly the fastest route to a crashed E2, and a scenario with zero human data. AI use with research peptides is likewise entirely uncharacterised.
• Interaction between AI use and the two things most monitored on testosterone — hematocrit/erythrocytosis and lipids — is unstudied in this population. Leder 2004 found hematocrit unchanged on AI monotherapy in 37 elderly men over 12 weeks; that is the extent of the data, and it says nothing about a man on injected testosterone.
• Mood and cognition on a suppressed estradiol in men are unmeasured. Loves 2013 assessed psychological endpoints and found no effect at low-dose letrozole in obese hypogonadal men, but no trial has examined depression, anxiety or cognition in men whose estradiol was driven below 10 pg/mL while on exogenous testosterone.
• No clinical practice guideline endorses this use. The Endocrine Society's testosterone therapy guideline (Bhasin 2018, PMID 29562364) specifies a monitoring plan of symptoms, adverse effects, compliance, serum testosterone, hematocrit and prostate cancer risk — routine estradiol measurement and aromatase inhibitor co-prescription are not part of the recommended standard of care. AI use in men has been, in the words of a 2021 review, 'off-label and empirical for decades' (PMID 34871401).

Sources

Every identifier below was checked to resolve to a real record before publication. Citations retrieved from PubMed and DailyMed.

1. The New England journal of medicine (2013) — PubMed 24024838 Androgen deficiency accounted for decreases in lean mass, muscle size, and strength; estrogen deficiency primarily accounted for increases in body fat; and both contributed to the decline in sexual function.
2. The Journal of clinical investigation (2016) — PubMed 26901812 QCT spine BMD fell substantially in all testosterone-dose groups in which aromatization was also suppressed, and this decline was independent of testosterone dose. Estradiol deficiency disrupted cortical microarchitecture at peripheral sites.
3. The Journal of clinical endocrinology and metabolism (2009) — PubMed 19820017 In older men, aromatase inhibition increases testosterone levels, decreases estradiol levels, and appears to decrease BMD. Aromatase inhibition does not improve skeletal health in aging men with low or low normal testosterone levels.
4. Clinical endocrinology (2009) — PubMed 18616708 Anastrozole administration normalized androgen production in older hypogonadal men and decreased estradiol production modestly. These alterations did not improve body composition or strength.
5. The Journal of clinical endocrinology and metabolism (2004) — PubMed 15001605 Serum LH levels increased from 5.1 +/- 4.8 to 7.9 +/- 6.5 U/liter and from 4.1 +/- 1.6 to 7.2 +/- 2.8 U/liter in groups 1 and 2, respectively... Scores for hematocrit, MOS Short-Form Health Survey, International Index of Erectile Function, and American Urologi
6. The Journal of clinical endocrinology and metabolism (2001) — PubMed 11397902 Despite the limited decrease of estrogen and the increase in testosterone, C-telopeptide of type 1 collagen showed a progressive significant increase of 11%, 24%, and 33% (P for trend = 0.033) above baseline at 3, 6, and 9 weeks, respectively.
7. Andrology (2015) — PubMed 26588809 At 12 months, TT improved the primary outcome of lumbar spine BMD (p < 0.01)... In summary, this proof-of-concept study confirms that aromatization of T is required for maintaining BMD in older men with low-T levels.
8. Epilepsy & behavior : E&B (2010) — PubMed 20096638 Normalization of sexual function (S-score) occurred with greater frequency in the T-A (72.2%) than in the T-P (47.4%) group, but the difference was not statistically significant. T-A resulted in significantly lower estradiol levels and S-scores correlated inve
9. European journal of endocrinology (2013) — PubMed 23949882 All patients started on one tablet of 2.5 mg/week, with subsequent dose escalation every month until a serum total testosterone of 20 nmol/l was reached... Despite a marked rise in serum testosterone, low-dose aromatase inhibition had no somatic or psychologic
10. The Journal of clinical endocrinology and metabolism (2016) — PubMed 27300575 When adjacent estradiol level groups in cohort 1 were compared, the largest difference in VMS incidence was observed between the 5-9.9 and 10-14.9 pg/mL groups (38% vs 16%, P < .001)... Estradiol deficiency is the key mediator of VMS in hypogonadal men.
11. The Journal of clinical endocrinology and metabolism (2003) — PubMed 14671195 Exemestane suppressed plasma estradiol comparably with either dose [25 mg, 38% (P <or= 0.002); 50 mg, 32% (P <or= 0.008)], with a reciprocal increase in testosterone concentrations (60% and 56%; P <or= 0.003 for both)... Long-term efficacy and safety will need
12. Journal of bone and mineral research (2008) — PubMed 18518773 The inverse relation between serum E2 and fracture risk was nonlinear with a strong relation <16 pg/ml for E2 and 0.3 pg/ml for fE2. In conclusion, older Swedish men with low serum E2 and high SHBG levels have an increased risk of fractures.
13. The Journal of clinical endocrinology and metabolism (2009) — PubMed 19584177 Men with the lowest bioE2 (<11.4 pg/ml) or highest SHBG (>59.1 nm) had greater risk of all nonvertebral fractures [adjusted hazard ratio (HR) [95% confidence interval]: 1.5 (1.2-1.9) and 1.4 (1.1-21.8), respectively]. Men with the lowest bioT (<163.5 ng/dl) ha
14. Nature reviews. Endocrinology (2009) — PubMed 19707181 At presentation, all men with aromatase deficiency have tall stature, delayed bone maturation, osteopenia or osteoporosis and eunuchoid skeletal proportions... Transdermal estradiol treatment at a daily dose of about 25 microg might be adequate for lifelong re
15. The world journal of men's health (2025) — PubMed 39344113 Limited data on the testosterone:estradiol ratio in men exists, particularly due to the scan data on concurrent estradiol values in men receiving testosterone therapy or aromatase inhibitors. Nonetheless, there seems to be a range of apparently beneficial valu
16. Anastrozole — US Prescribing Information (DailyMed) Approved indication: Breast cancer in postmenopausal women only: adjuvant treatment of hormone receptor-positive early breast cancer; first-line treatment of HR-positive or HR-unknown locally advanced/metastatic breast ca
17. Exemestane — US Prescribing Information (DailyMed) Approved indication: Breast cancer in postmenopausal women only: adjuvant treatment of ER-positive early breast cancer after 2-3 years of tamoxifen, to complete five years of adjuvant hormonal therapy; and advanced breast
18. Letrozole — US Prescribing Information (DailyMed) Approved indication: Breast cancer in postmenopausal women only: adjuvant treatment of HR-positive early breast cancer; extended adjuvant treatment after 5 years of tamoxifen; first- and second-line treatment of HR-positi

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