Anastrozole

Why there is no syringe

There is no syringe. Anastrozole is a single 1 mg film-coated tablet and the label supplies no other strength, which is the practical problem behind most off-label male use of it: men are routinely told to take a fraction of a tablet two or three times a week, and there is no labelled basis for any of those fractions because there is no labelled male dose at all. Splitting an oncology tablet to approximate a dose nobody has approved is not a conversion this page will perform. What can be said with a citation is what the AUA guideline says about the whole class, and what the ARIMIDEX label’s own controlled trial in males found — both are below.

Administration

1. 1

   Read the indication, because it is oncology

   ARIMIDEX is “an aromatase inhibitor indicated for: adjuvant treatment of postmenopausal women with hormone receptor-positive early breast cancer; first-line treatment of postmenopausal women with hormone receptor-positive or hormone receptor-unknown locally advanced or metastatic breast cancer; treatment of advanced breast cancer in postmenopausal women with disease progression following tamoxifen therapy.” That is the whole of section 1. Using it to suppress estradiol in a man on testosterone is off-label prescribing of a breast cancer drug.

2. 2

   Know the one controlled male trial that is on the label — and that it failed

   Section 8.4 describes “a randomized, double-blind, placebo-controlled, multi-center study” that “enrolled 80 boys with pubertal gynecomastia aged 11 to 18 years”, randomised to ARIMIDEX 1 mg daily or placebo. “After 6 months of treatment there was no statistically significant difference in the percentage of patients who experienced a ≥50% reduction in gynecomastia (primary efficacy analysis).” Secondary analyses agreed. Serum estradiol fell 15.4% on anastrozole against 4.5% on placebo. The label’s own summary: “The efficacy of ARIMIDEX in the treatment of pubertal gynecomastia in adolescent boys … has not been demonstrated.” That is the highest-quality male evidence inside the label, and it is negative.

3. 3

   Treat the guideline position as the only citable guidance for male use

   The Endocrine Society's 2018 testosterone guideline does not mention aromatase inhibitors at all — the words “aromatase”, “anastrozole” and “letrozole” appear nowhere in its 30 pages. The AUA guideline does address them, conditionally and with a caveat: “Clinicians may use aromatase inhibitors, human chorionic gonadotropin, selective estrogen receptor modulators, or a combination thereof in men with testosterone deficiency desiring to maintain fertility. (Conditional Recommendation; Evidence Level: Grade C)” Its discussion then adds: “Clinicians should understand that of these agents, only hCG has been approved by the FDA for use in males.” Note the context those statements sit in: preserving fertility, not managing estradiol on testosterone therapy.

4. 4

   Do not let it near anyone who could become pregnant

   Section 8.1: “Based on findings from animal studies and its mechanism of action, ARIMIDEX may cause fetal harm when administered to a pregnant woman.” In rabbits anastrozole caused pregnancy failure at doses equal to or greater than 16 times the recommended human dose on a mg/m² basis. The label directs that females of reproductive potential use effective contraception during therapy and for at least three weeks after the last dose.

5. 5

   If it is prescribed, test at the right interval

   The AUA guideline gives a timing point rather than a protocol: “Patients using anastrozole, clomiphene citrate, or hCG should be tested no earlier than four weeks.” Testing sooner measures a system that has not yet settled.

Storage

• —“Store at controlled room temperature, 20 to 25°C (68 to 77°F) [see USP]” — quoted from section 16 of the ARIMIDEX label.
• —Supplied as 1 mg tablets in bottles of 30. One strength only; the label describes no other.
• —Keep out of reach of children and of anyone who is or could become pregnant. The label requires contraception during treatment and for at least three weeks after the last dose in females of reproductive potential.

Monitoring

Frequently asked questions

Is anastrozole FDA-approved for men on testosterone?

No. ARIMIDEX (NDA 020541) is approved for adjuvant, first-line and second-line treatment of breast cancer in postmenopausal women. There is no male indication of any kind on the label, and no indication relating to testosterone therapy, estradiol management or gynecomastia in adults. Prescribing it to a man on TRT is off-label use of an oncology drug.

Has anastrozole ever been tested in males in a controlled trial?

Yes, and the result is printed on the label. Section 8.4 describes a randomised, double-blind, placebo-controlled, multi-centre study in 80 boys aged 11 to 18 with pubertal gynecomastia, randomised to 1 mg of anastrozole daily or placebo for six months. There was no statistically significant difference on the primary endpoint — the proportion achieving a ≥50% reduction in gynecomastia — and the secondary analyses agreed. Serum estradiol fell 15.4% on drug versus 4.5% on placebo. The label concludes that efficacy “has not been demonstrated”. Those boys are not men on TRT, but it is the only controlled male trial the label contains, and it is negative.

What does the Endocrine Society guideline say about aromatase inhibitors with TRT?

Nothing. This is worth stating plainly because it is widely assumed otherwise. The full text of the 2018 Endocrine Society clinical practice guideline on testosterone therapy in men with hypogonadism — 30 journal pages, pp. 1715–1744 — contains no occurrence of “aromatase”, “anastrozole” or “letrozole”. It carries a technical remark on clomiphene and nothing on aromatase inhibitors. The guideline that does address them is the AUA’s, and only in the context of preserving fertility.

What does the AUA guideline say?

Statement 27, verbatim: “Clinicians may use aromatase inhibitors, human chorionic gonadotropin, selective estrogen receptor modulators, or a combination thereof in men with testosterone deficiency desiring to maintain fertility. (Conditional Recommendation; Evidence Level: Grade C)” Its accompanying discussion explains that exogenous testosterone suppresses intratesticular testosterone production, which is why alternatives are used at all, and then draws the distinction that matters: “Clinicians should understand that of these agents, only hCG has been approved by the FDA for use in males.” So the AUA permits an aromatase inhibitor conditionally, at the weakest evidence grade it uses, in a specific population — men who want to preserve fertility — while stating that the drug is not FDA-approved for males.

What dose of anastrozole should a man take?

This page does not answer that, and the reason is not caution for its own sake. There is one approved strength, 1 mg, and one approved schedule, once a day, both belonging to a breast cancer indication in postmenopausal women. Every male regimen in circulation — a quarter tablet twice a week, half a tablet with each injection — is derived from practice rather than from any label or trial. No regulator has evaluated any of them. If a prescriber recommends one, the honest framing is that it is off-label, that the evidence base for it in men is thin, and that estradiol is not simply a problem to be minimised.

Sources

1. [1]FDA label — ARIMIDEX (anastrozole) tablet, ANI Pharmaceuticals, Inc. (DailyMed SPL acbfaaa9-503c-4691-9828-76a7146ed6de; Drugs@FDA NDA 020541, initial US approval 1995). Source of the three breast cancer indications, the 1 mg once-daily dose, the section 8.4 pubertal gynecomastia trial and its negative result, the embryo-fetal toxicity statement and the bone and cholesterol warnings
2. [2]Mulhall JP, Trost LW, Brannigan RE, et al. Evaluation and Management of Testosterone Deficiency: AUA Guideline. J Urol. 2018;200(2):423–432 (PMID 29601923). Statement 27 and its discussion — the source of both quoted sentences, including “Clinicians should understand that of these agents, only hCG has been approved by the FDA for use in males.”
3. [3]Bhasin S, Brito JP, Cunningham GR, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018;103(5):1715–1744 (PMID 29562364). Source of the clomiphene technical remark under recommendation 2.1. Read in full on 2026-09-19: the guideline contains no mention of aromatase inhibitors, anastrozole or letrozole
4. [4]Drugs@FDA: FDA-Approved Drugs — used to confirm NDA 020541 and that no approved anastrozole application carries a male indication

Related

• Testosterone cypionate — the labelled comparator
• TRT — how it works and what it costs
• Compare online TRT providers
• Estradiol on TRT — reframing the estrogen conversation
• hCG — the one with an approved male use