Anastrozole

Brand names: Arimidex. Also called Arimidex, aromatase inhibitor, AI.

Aromatase inhibitor (nonsteroidal) 6 graded human studies Last checked Oct 23, 2026

The short answer

From 9 sources

Anastrozole blocks aromatase, the enzyme that turns testosterone into estradiol. Its label covers breast cancer in postmenopausal women, where it lowers recurrence and, in high-risk women, new cancers. In men it is prescribed to raise testosterone or lower estradiol, but trials show it can thin the spine, and estradiol matters for men's body fat and sexual function.

Key facts

Form: 1 mg oral tablet¹
Labelled for: Breast cancer in postmenopausal women¹
Label dose: 1 mg once daily¹
Breast cancer prevention: Halved new cancers in high-risk women (IBIS-II)⁴
In men: In men: raises testosterone, lowers estradiol^5,6
Bone: Spine density fell vs placebo in older men⁵

Which anastrozole products are approved, and for what?

Anastrozole 1 mg tablets (Arimidex and generics) are FDA-approved prescription drugs for breast cancer in postmenopausal women, as adjuvant treatment and for advanced disease. No boxed warning. Use in men, alone or alongside testosterone therapy, is off-label and is not covered by the label.

Status by form

2 forms

Anastrozole products and their regulatory status
Form	Status	Approved for	Boxed warning
Oral tablets 1 mgArimidex, generics	FDA-approved prescription drug¹	Postmenopausal women with hormone receptor-positive early breast cancer (adjuvant) and advanced or metastatic breast cancer	None on the current label
Use in men (low testosterone, high estradiol on TRT)	Off-label prescribing of the same tablets; not FDA-approved for men¹	None	Not applicable

What is anastrozole and how does it work?

Aromatase converts testosterone into estradiol in fat, muscle, brain and other tissues. Anastrozole blocks that step, so estradiol falls. After menopause most estrogen comes from this conversion, which is why it treats hormone-sensitive breast cancer. In men lower estradiol lets the pituitary release more LH, so testosterone rises.

How it works

Aromatase inhibitor (nonsteroidal)

Anastrozole binds aromatase (CYP19) and blocks it, so androgens such as testosterone are no longer converted to estradiol. In postmenopausal women this removes most of the body's estrogen. In men, less estradiol weakens the brake on the pituitary, LH rises and the testes make more testosterone.^1,6

What doses are on the label and in studies?

The label dose is 1 mg once daily, with or without food, for breast cancer; adjuvant treatment ran five years in the ATAC trial. In men, trials used 1 mg daily or 1 mg twice a week for 12 weeks to a year. Doses for men are study doses, not label doses.

Label and study amounts, not a dose for you. A prescriber decides whether and how much; these show what the labels print and what trials used.

Doses by use

4 rows

Anastrozole doses as labelled or studied
Use	Form	Amount and schedule	Basis
Breast cancer (adjuvant or advanced), postmenopausal women	Tablets	1 mg once daily, with or without food; adjuvant treatment ran 5 years in ATAC¹	Label
Older men with low or borderline testosterone	Tablets	1 mg daily or 1 mg twice weekly for 12 weeks⁶	Study
Older men with low testosterone	Tablets	1 mg daily for 1 year⁵	Study
Hypogonadal infertile men	Tablets	1 mg daily for 12 weeks, compared with clomiphene 25 mg daily⁷	Study

What is anastrozole combined with?

In breast cancer anastrozole replaces tamoxifen rather than joining it: the label says not to combine them. In men it is compared with clomiphene as a way to raise testosterone while keeping sperm, and it is sometimes added to testosterone therapy to lower estradiol, which trials show has costs.

Anastrozole and tamoxifen

Both treat hormone-sensitive breast cancer.

The combination arm of ATAC was stopped for lack of benefit, and tamoxifen lowered anastrozole levels by 27%. The label says not to use them together. On its own, anastrozole improved disease-free survival over tamoxifen at ten years.^1,3

Anastrozole added to testosterone therapy

To lower the estradiol that testosterone produces.

No trial in this listing tested anastrozole as an add-on to testosterone for symptoms. A controlled NEJM study that blocked estradiol in men found estrogen deficiency drove gains in body fat and contributed to falling sexual function, and a one-year trial of anastrozole in older men lowered spine bone density.^2,5

Checked: Estradiol, and bone density with longer use.

Anastrozole or clomiphene, in men

Both raise testosterone while keeping sperm production.

In a 12-week randomized trial of 26 hypogonadal infertile men, clomiphene raised testosterone more (571 vs 408 ng/dL) but estradiol rose on clomiphene and fell on anastrozole; semen measures did not change in either group.⁷

How does anastrozole relate to fertility and pregnancy?

Anastrozole can harm a fetus, so the label asks for effective contraception in women who could become pregnant. In men it raises LH and testosterone without the sperm suppression of testosterone therapy; a small trial in infertile men found semen measures unchanged after 12 weeks.

Fertility, pregnancy and breastfeeding

Pregnancy: Anastrozole may cause fetal harm; the label advises effective contraception during treatment for women who could become pregnant.¹
Sperm in men: In a 12-week trial in hypogonadal infertile men, anastrozole 1 mg daily raised testosterone without changing semen measures.⁷

What gets checked during treatment?

The label suggests monitoring bone density and cholesterol, both of which worsened on anastrozole in the ATAC trial. In men, trials tracked testosterone, estradiol and LH after about three months, and the one-year trial also measured spine bone density, which fell compared with placebo.

Monitoring

3 checks

What is monitored on anastrozole
Check	When	Why
Bone density	During treatment (the label suggests considering it)^1,5	Spine and hip density fell on anastrozole in ATAC, and spine density fell in older men
Cholesterol	During treatment (the label suggests considering it)¹	High cholesterol was reported in 9% on anastrozole vs 3.5% on tamoxifen
Testosterone, estradiol and LH (men)	After 3 months in the trials, then periodically^5,6	To see the rise in testosterone and how far estradiol fell

What does the research show?

In ATAC, 6,241 women with early breast cancer had better disease-free survival on anastrozole than tamoxifen over ten years. In IBIS-II it halved breast cancers in high-risk women. In older men it raised testosterone but lowered spine bone density, and an NEJM study showed estradiol drives men's body fat and sexual function.

Every graded row

6 rows from 6 studies

Form tested

Showing 6 of 6 outcomes. Human trials only.

Grade ADisease-free survival vs tamoxifen (early breast cancer)Breast cancerImprovedRCT
n 6,241 · 2010

Better disease-free survival (HR 0.91), time to recurrence (HR 0.84) and time to distant recurrence (HR 0.87) than tamoxifen.

Design: RCT, n = 6,241, 2010
Population: Postmenopausal women with early-stage breast cancer
Form: Tablets
Dose: 1 mg daily for 5 years
Duration: Median follow-up 120 months
Source: Reference 3 · PubMed 21087898 (opens PubMed)

Grade ANew breast cancers in high-risk womenBreast cancer preventionImprovedRCT
n 3,864 · 2014

Breast cancer in 2% vs 4% on placebo (HR 0.47); predicted 7-year incidence 2.8% vs 5.6%.

Design: RCT, n = 3,864, 2014
Population: Postmenopausal women at high risk of breast cancer
Form: Tablets
Dose: 1 mg daily
Duration: Median 5 years
Source: Reference 4 · PubMed 24333009 (opens PubMed)

Grade BBody fat and sexual function when estradiol is blockedEstradiol (men)WorsenedRCT
n 400 · 2013

Estrogen deficiency mainly accounted for increases in body fat; both androgen and estrogen deficiency contributed to falling sexual function.

Design: RCT, n = 400, 2013
Population: Healthy men 20 to 50 with testosterone and estradiol suppressed, some given anastrozole
Form: Testosterone gel with or without anastrozole
Dose: Graded testosterone doses
Duration: 16 weeks
Source: Reference 2 · PubMed 24024838 (opens PubMed)

Grade BSpine bone density in older menBone (men)WorsenedRCT
n 69 · 2009

Testosterone rose from 319 to 524 ng/dL, but spine bone density fell vs placebo; bone turnover markers did not change.

Design: RCT, n = 69, 2009
Population: Men 60 and older with low or borderline testosterone
Form: Tablets
Dose: 1 mg daily
Duration: 1 year
Source: Reference 5 · PubMed 19820017 (opens PubMed)

Grade BTestosterone in older menTestosterone (men)ImprovedRCT
n 37 · 2004

Total testosterone rose from 343 to 572 ng/dL on daily dosing; estradiol fell from 26 to 17 pg/mL; no change in quality of life or sexual function scores.

Design: RCT, n = 37, 2004
Population: Men 62 to 74 with testosterone under 350 ng/dL
Form: Tablets
Dose: 1 mg daily or 1 mg twice weekly
Duration: 12 weeks
Source: Reference 6 · PubMed 15001605 (opens PubMed)

Grade CTestosterone vs clomipheneTestosterone (men)ImprovedRCT
n 26 · 2015

Testosterone rose to 408 ng/dL vs 571 on clomiphene; estradiol fell and the testosterone-to-estradiol ratio improved; semen unchanged.

Design: RCT, n = 26, 2015
Population: Hypogonadal infertile men, testosterone under 350 ng/dL
Form: Tablets
Dose: 1 mg daily (vs clomiphene 25 mg)
Duration: 12 weeks
Source: Reference 7 · PubMed 26176805 (opens PubMed)

What are the side effects and risks?

In ATAC, the commonest effects were hot flashes, joint pain and arthritis, high blood pressure, mood changes and weakness. Compared with tamoxifen it caused more fractures, osteoporosis and high cholesterol, and in women with heart disease more ischemic events. It can harm a pregnancy. Allergic reactions are the only contraindication.

Regulatory status. Anastrozole tablets are FDA-approved prescription drugs for breast cancer in postmenopausal women, with no boxed warning; use in men is off-label.¹

Side effects in the ATAC trial, compared with tamoxifen

From the label; anastrozole vs tamoxifen

Common side effects of anastrozole
Side effect	Rate: drug vs placebo
Hot flashes	36% vs 41% on tamoxifen (ATAC)¹
Arthritis	17% vs 14%¹
Joint pain	15% vs 11%¹
High blood pressure	13% vs 11%¹
Osteoporosis	11% vs 7%¹
Fracture	10% vs 7%¹
High cholesterol	9% vs 3.5%¹

Warnings on the label

Bone loss and fractures: Spine and hip bone density fell on anastrozole while it rose on tamoxifen; fractures were more frequent.¹
Ischemic heart events: In women with existing ischemic heart disease, 17% had ischemic events on anastrozole vs 10% on tamoxifen.¹
Cholesterol: Total cholesterol can rise; the label suggests monitoring.¹
Estrogen and men: In men, estradiol deficiency increased body fat and contributed to lower sexual function in a controlled study.²

Who should not use it (label contraindications)

Hypersensitivity to anastrozole or its ingredients, including anaphylaxis, angioedema and urticaria.¹

What does anastrozole interact with?

The label lists two: tamoxifen, which lowered anastrozole levels by 27% and gave no added benefit, so the two are not combined; and estrogen-containing products, which may reduce its effect. In men on testosterone, lowering estradiol is the intended effect, with the bone and sexual-function costs described above.

Interactions

2 agents

Published interactions with anastrozole
With	What happens	Evidence
TamoxifenTamoxifen	Lowered anastrozole levels by 27% with no added benefit; not used together.¹	Drug label
Estrogen-containing productsEstradiol, combined hormone therapy	May reduce anastrozole's effect.¹	Drug label

Check your own medicines and supplements in the Interaction map: every edge is sourced, and nothing you enter leaves your browser.

How do the options compare?

Anastrozole lowers estradiol, while clomiphene raises LH and testosterone by blocking estrogen feedback and lets estradiol rise. In a 12-week trial in infertile men, clomiphene raised testosterone more, but only anastrozole improved the testosterone-to-estradiol ratio significantly. Letrozole and exemestane are the other aromatase inhibitors on the market.

Anastrozole or clomiphene in men (12-week trial)

Anastrozole or clomiphene in men (12-week trial)
	Anastrozole 1 mg daily	Clomiphene 25 mg daily
How it works	Blocks conversion of testosterone to estradiol	Blocks estrogen feedback at the pituitary
Testosterone at 12 weeks	408 ng/dL	571 ng/dL
Estradiol	Fell	Rose
Semen measures	No significant change	No significant change

Which tools and services help with this?

Getting a prescription

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Frequently asked questions

Common questions

5 answers

Do men on TRT need anastrozole?

No trial in this listing shows symptom benefit from adding it. Blocking estradiol in men raised body fat and lowered sexual function in a controlled study, and a one-year trial in older men lowered spine bone density.^2,5

How much does anastrozole lower estradiol in men?

In older men, 1 mg daily lowered estradiol from 26 to 17 pg/mL over 12 weeks, and from 15 to 12 pg/mL in a one-year trial, while testosterone rose.^6,5

Is anastrozole better than tamoxifen for breast cancer?

After menopause, in the ATAC trial, anastrozole gave better disease-free survival and fewer recurrences over ten years, with more fractures and joint symptoms but fewer hot flashes.^3,1

Can anastrozole prevent breast cancer?

In high-risk postmenopausal women, IBIS-II found breast cancer in 2% on anastrozole vs 4% on placebo after a median five years.⁴

Is anastrozole the same as letrozole?

Both are nonsteroidal aromatase inhibitors with breast cancer labels; letrozole (Femara) is dosed at 2.5 mg daily and exemestane (Aromasin) is a steroidal inhibitor dosed at 25 mg.^1,8,9

Sources

Sources cited on this page

9 sources

1Arimidex (anastrozole) tablets: prescribing information (DailyMed, version effective 2026-03-02)Label ANI Pharmaceuticals, 2026
2Gonadal steroids and body composition, strength, and sexual function in menRCT N Engl J Med, 2013 · PMID 24024838
3Effect of anastrozole and tamoxifen as adjuvant treatment for early-stage breast cancer: 10-year analysis of the ATAC trialRCT Lancet Oncol, 2010 · PMID 21087898
4Anastrozole for prevention of breast cancer in high-risk postmenopausal women (IBIS-II)RCT Lancet, 2014 · PMID 24333009
5Effects of aromatase inhibition on bone mineral density and bone turnover in older men with low testosterone levelsRCT J Clin Endocrinol Metab, 2009 · PMID 19820017
6Effects of aromatase inhibition in elderly men with low or borderline-low serum testosterone levelsRCT J Clin Endocrinol Metab, 2004 · PMID 15001605
7A Randomized Prospective Double-Blind Comparison Trial of Clomiphene Citrate and Anastrozole in Raising Testosterone in Hypogonadal Infertile MenRCT J Sex Med, 2015 · PMID 26176805
8Femara (letrozole) tablets: prescribing information (DailyMed, version effective 2026-06-11)Label Novartis, 2026
9Aromasin (exemestane) tablets: prescribing information (DailyMed, version effective 2025-12-24)Label Pharmacia & Upjohn (Pfizer), 2025

What changed

Change log

2 entries

Dated changes to this page
Date	Change	Who and why
Oct 23, 2026	"Off-label" now appears only in the status box and the safety band; dose rows and key facts name the population instead of tagging it.	Writer: Kymata Health EditorialTrigger: Growth review (2026-10-23): status once or twice per page
Oct 23, 2026	Page created: label status and dose, off-label use in men, combinations with tamoxifen, testosterone and clomiphene, monitoring, six graded rows, ATAC side-effect rates and a comparison with clomiphene in men.	Writer: Kymata Health EditorialTrigger: Owner request (2026-10-22): hormone library

Kymata Health hormone library. Educational information only; it is not medical advice and does not replace a clinician. Doses are shown as labels and studies print them; a prescriber decides whether and how any hormone is used.