Peptide & Experimental

Anastrozole

Q: Does anastrozole raise testosterone in men?

Yes, in the lab. In 88 older men with low testosterone, the level rose from 11.2 to 18.2 nmol/l within 3 months. However, body composition and strength did not change over a year, and in an analysis with 69 men, lumbar spine bone density fell.

Q: Does anastrozole help against breast enlargement?

Not in the only randomized study. In 80 boys with breast enlargement during puberty, breast volume decreased by at least half in 38.5 percent on anastrozole and 31.4 percent on placebo, a difference without significance. No controlled studies were found on breast enlargement caused by anabolic steroids.

Q: What happens if estradiol is too low in men?

In a study with 400 healthy men, the increase in body fat was mainly due to estrogen deficiency, and it contributed to the decline in sexual function. In older men, lumbar spine bone density fell on anastrozole, while it rose slightly on placebo.

Q: Is anastrozole approved in Germany?

Yes, as a medicine for hormone receptor-positive breast cancer in postmenopausal women, at a dose of 1 mg once daily. It is prescription-only. Use in men is not approved.

Q: Is anastrozole banned in sport?

Yes. The World Anti-Doping Agency lists aromatase inhibitors under S4.1, prohibited at all times. In Germany, anastrozole is listed in the annex of the Anti-Doping Act; this means that acquisition and possession of a not insignificant quantity for the purpose of doping in sport are also prohibited.

Aromatase inhibitor (non-steroidal), prescription-only · Arimidex, anastrozole

Anastrozole is an approved breast cancer medicine and very well supported in that use: in a study with 9,366 postmenopausal women it performed better than tamoxifen. Outside its approval, men use it to lower estradiol or raise testosterone. That works in the lab, but in a one-year study it brought neither muscle mass nor strength; bone density fell.

What anastrozole is

Anastrozole is a non-steroidal aromatase inhibitor and prescription-only. It is approved for postmenopausal women with hormone receptor-positive breast cancer, in the advanced stage and as adjuvant treatment after surgery. The European Medicines Agency harmonized indications and dose across the EU in 2011.

What is rated is the state of knowledge. For the approved use it is very good. For use in men – with low testosterone, for “estrogen control” alongside anabolic steroids or against breast enlargement – there are few studies, and they mainly show changed lab values.

How it works

The enzyme aromatase converts androgens such as testosterone and androstenedione into estrogens. Anastrozole inhibits it selectively. At the approved dose, according to the US prescribing information, estradiol in postmenopausal women falls by about 70 percent within 24 hours and by about 80 percent after 14 days. The production of adrenal hormones such as cortisol and aldosterone remains unaffected.

In men, less estradiol loosens the brake on the pituitary gland: it releases more LH, and the testes produce more testosterone. But estradiol is not a waste product in men. In a study with targeted hormone control in 400 healthy men, the increase in body fat was mainly due to estrogen deficiency, and the decline in sexual function to testosterone and estrogen deficiency together.

What is well supported

Protection against recurrence in breast cancer. In ATAC, 9,366 postmenopausal women received five years of anastrozole or tamoxifen. After a median of 68 months, disease-free survival was longer on anastrozole: 575 versus 651 events, hazard ratio 0.87 (95 percent confidence interval 0.78 to 0.97). Contralateral breast cancer occurred in 35 versus 59 women.
The lab effect in men. In 88 older men with low testosterone, the level rose on anastrozole from 11.2 to 18.2 nmol/l within 3 months and remained above that of the placebo group over a year. Estradiol fell from 55.8 to 42.2 pmol/l.
The bone risk is measured, not assumed. In women in the ATAC bone substudy and in men in a one-year randomized study, bone density fell on anastrozole.
Reviewed by the authorities. Indications, dose and contraindications have been uniformly regulated in the EU since the European Commission’s decision of May 19, 2011.

What the studies show

ATAC: anastrozole versus tamoxifen

Howell 2005 (Lancet) compared five years of anastrozole with five years of tamoxifen in 9,366 postmenopausal women with localized breast cancer. After a median of 68 months, anastrozole prolonged disease-free survival (hazard ratio 0.87; 0.78 to 0.97; p = 0.01) and time to recurrence (0.79; 0.70 to 0.90; p = 0.0005), and reduced distant metastases (0.86; 0.74 to 0.99) and contralateral breast cancer (35 versus 59 cases). Fewer women discontinued than on tamoxifen. Gynecological complaints and vascular events were less frequent, joint pain and fractures more frequent.

Burnett-Bowie 2009: one year in older men

Burnett-Bowie 2009 (Clin Endocrinol) treated 88 men aged 60 and over with low or borderline testosterone and symptoms for one year, double-blind, with anastrozole or placebo. Testosterone rose from 11.2 to 18.2 nmol/l after 3 months (p<0.0001), bioavailable testosterone from 2.7 to 5.4 nmol/l; thereafter the values declined somewhat but remained above placebo. Estradiol fell from 55.8 to 42.2 pmol/l. Body composition, measured by CT and DXA, and strength did not change, nor did PSA, prostate symptoms, hematocrit and blood lipids.

The men’s bones

An analysis by the same research group with 69 men aged 60 and over (J Clin Endocrinol Metab 2009) measured bone density. Testosterone rose from 319 to 524 ng/dl, estradiol fell from 15 to 12 pg/ml. Lumbar spine bone density fell on anastrozole from 1.121 to 1.102 g/cm², while on placebo it rose from 1.180 to 1.189 g/cm²; the difference was significant (p = 0.0014). Other measurement sites showed the same direction, but not significantly. The authors conclude that aromatase inhibition does not improve bone health in older men.

Finkelstein 2013: what estradiol does in men

Finkelstein 2013 (NEJM) suppressed the body’s own hormone production in 400 healthy men aged 20 to 50 with goserelin and then gave placebo or testosterone gel at four doses over 16 weeks; half also received anastrozole. Androgen deficiency explained the loss of lean mass, muscle size and strength. Estrogen deficiency mainly explained the increase in body fat. Both contributed to the decline in sexual function.

Plourde 2004: breast enlargement during puberty

Plourde 2004 (J Clin Endocrinol Metab) treated 80 boys aged 11 to 18 with pubertal gynecomastia for six months, double-blind, with anastrozole or placebo. A decrease in breast volume of at least half was achieved by 38.5 versus 31.4 percent (odds ratio 1.51; 0.50 to 4.84; p = 0.47). The ratio of testosterone to estradiol rose by a median of 166 versus 39 percent. The hormone profile shifted; breast volume did not change differently than on placebo.

Where the data stop

Alongside anabolic steroids. No controlled studies on lowering estradiol during anabolic steroid use were found in the research. Whether anastrozole prevents side effects there and what additional harm it causes has not been studied.
Breast enlargement in adults. The only randomized study was conducted in boys during puberty and found no difference from placebo. No comparable study was found for men with breast enlargement caused by administered hormones.
Benefit in low testosterone. A higher testosterone value is a surrogate marker. Muscle mass, strength and body composition did not change over a year.
Long-term consequences in men. The longest study in men lasted one year. Whether the measured decrease in bone density leads to fractures over years has not been studied in men.

Status, approval and legal

Anastrozole is approved as Arimidex in all EU member states, via mutual recognition and national procedures. Following a harmonization procedure by the European Medicines Agency, uniform indications have applied since the European Commission’s decision of May 19, 2011: hormone receptor-positive advanced breast cancer in postmenopausal women and adjuvant treatment of early breast cancer, including after two to three years of tamoxifen. The approved dose is 1 mg once daily; adjuvant treatment lasts five years. Contraindications are pregnancy and breastfeeding; tamoxifen and estrogen-containing therapies should not be given together with anastrozole.

In Germany, anastrozole is prescription-only (Annex 1 of the German Prescription Medicines Ordinance). Use in men is not approved.

In sport, anastrozole is on the World Anti-Doping Agency’s 2026 Prohibited List under S4.1 aromatase inhibitors and is prohibited at all times, in and out of competition. In Germany it is listed in the annex of the Anti-Doping Act among the hormone and metabolic modulators; this means that acquisition and possession of a not insignificant quantity for the purpose of doping in sport are also prohibited.

Safety

The best-studied risk concerns the bones. In the ATAC bone substudy, bone density at the lumbar spine and hip fell on anastrozole and rose on tamoxifen; fractures were more frequent. In older men, lumbar spine bone density fell within one year. The US prescribing information recommends considering a bone density measurement. If you are prescribed anastrozole, this question belongs in the conversation.

In women in ATAC, cholesterol was elevated more often than on tamoxifen, 9 versus 3.5 percent. Women with pre-existing coronary heart disease had more ischemic cardiovascular events, 17 versus 10 percent. In the one-year study in men, blood lipids remained unchanged. Joint pain was more frequent in ATAC than on tamoxifen.

One risk that was specifically studied in men is estradiol that is too low. In 400 healthy men, the increase in body fat was mainly due to estrogen deficiency, and it contributed to the decline in sexual function. An estradiol value as low as possible is therefore not a sensible goal in men.

BK-Score Supported, with caveats

Human evidence		7
Mechanism		9
Safety data		8
Hype gap		4
Track record of use		9

Evidence 7, because the approved use is supported by very large randomized trials – in ATAC with 9,366 women, anastrozole prolonged disease-free survival compared with tamoxifen (HR 0.87) – but the randomized studies in men show only higher testosterone values, no gain in muscle mass or strength and no advantage over placebo in pubertal breast enlargement. Mechanism 9, because aromatase inhibition is well described and a study with targeted hormone control in 400 men has broken down which effects in men are due to estradiol. Safety 8, because bone, lipid and cardiac data in women are available from large studies, and bone density data in men from a one-year randomized study; that means well studied, not harmless. Hype 4, because the scene promises “estrogen control” and a testosterone boost that in studies remained a lab value without measurable benefit. Use 9, because anastrozole is widely used in breast cancer therapy and in ATAC alone was given for five years to thousands of women. Direction mixed: proven benefit within the approval, no proven benefit in men, plus a bone risk when estradiol falls.

The score rates the state of knowledge, not the substance. “Safety data 9” means well studied – not harmless. “Track record of use 9” means used long and widely – that is not proof of efficacy.
Subjective assessment by Biohacking Kompakt based on published scoring rules – not a scientific rating and not a medical recommendation. Rules and all ratings (German)

Frequently asked questions about anastrozole

Does anastrozole raise testosterone in men?