Key points
- Anastrozole (Arimidex) is a tablet that lowers the making of female hormone (oestrogen) in women after menopause and is used for hormone receptor-positive breast cancer [1].
- In a large study, anastrozole prevented the cancer from coming back better than tamoxifen, and this advantage lasted up to 10 years [2, 3].
- One 1 mg tablet a day, with or without food [1]. Take it every day until the end of the course (why?).
- In women who have not yet gone through menopause, it does not work on its own [1].
- The main side effects: hot flushes, joint pain, weaker bones and higher cholesterol; tell your doctor if you have heart disease [1].
What is this drug and how does it work?
For physicians
Anastrozole (brand name Arimidex) is a selective, non-steroidal aromatase inhibitor first approved in the United States in 1995 [1].
The growth of many breast cancers is stimulated by oestrogen. In postmenopausal women, oestrogen is made mainly from adrenal androgens by the enzyme aromatase. By inhibiting this enzyme, anastrozole markedly lowers blood oestradiol without a meaningful effect on corticosteroid or aldosterone synthesis [1].
Because the ovaries are the main source of oestrogen before menopause, anastrozole alone is not effective in premenopausal women [1]. In trials such as NATALEE, premenopausal women also received ovarian suppression (goserelin) with the aromatase inhibitor [4].
Which cancers is it used for?
Your doctor may prescribe this drug for:
- Early hormone receptor-positive breast cancer [1, 2, 3]
- Advanced or spread breast cancer that is hormone receptor-positive or of unknown receptor status [1]
- Advanced breast cancer [1]
- High-risk early hormone receptor-positive, HER2-negative breast cancer [4]
- Prevention in high-risk women after menopause (not an FDA-labelled use) [1, 5]
Whether it suits you depends on the type and stage of your cancer and your test results. Details of each use are in the section for doctors.
For physicians
| Cancer | Setting | Key evidence |
|---|---|---|
| Early hormone receptor-positive breast cancer | Adjuvant treatment after surgery [1] | ATAC trial in 9366 women: 3-year disease-free survival 89.4% with anastrozole vs 87.4% with tamoxifen (HR 0.83) [2]. At 10-year follow-up in hormone receptor-positive patients: HR for disease-free survival 0.86 and for time to recurrence 0.79 [3]. |
| Advanced or metastatic breast cancer, hormone receptor-positive or unknown | First-line treatment [1] | In trial 0030, median time to progression was 11.1 months with anastrozole vs 5.6 months with tamoxifen; in trial 0027 the two drugs gave similar results (8.2 vs 8.3 months) [1]. |
| Advanced breast cancer | Treatment after progression on tamoxifen [1] | Approved in the FDA label; patients with ER-negative disease and those who had not responded to tamoxifen rarely responded [1]. |
| High-risk early HR-positive/HER2-negative breast cancer | Endocrine backbone with ribociclib in adjuvant treatment | NATALEE trial: adding ribociclib to letrozole or anastrozole raised 3-year invasive disease-free survival from 87.1% to 90.4% (HR 0.75) [4]. |
| Prevention in high-risk postmenopausal women (not an FDA-labelled use) | Lowering the risk of developing breast cancer | IBIS-II trial in 3864 women: lower breast cancer incidence with 5 years of anastrozole (2% vs 4%; HR 0.47) [5]. This use is not listed in the FDA label for Arimidex [1]. |
In adjuvant treatment the optimal duration is not known; in the ATAC trial anastrozole was given for 5 years [1].
How it is taken and dosing
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- Anastrozole is a tablet taken once a day; you can take it with or without food [1].
- Keep taking it for as long as your doctor says and never stop it on your own [1].
- If you forget a dose, take it as soon as you remember; if it is nearly time for the next dose, skip the missed one and never take two doses together [1].
- If you still have periods or are not sure you have gone through menopause, be sure to tell your doctor [1].
- Dose adjustment: not needed in renal impairment, older adults or mild to moderate hepatic impairment; severe hepatic impairment has not been studied. The label defines no dose reduction levels [1].
- Contraindication: hypersensitivity to the drug or its components (anaphylaxis, angioedema, urticaria) [1].
For physicians
Standard dosing (FDA label)
Adjuvant anastrozole
Early hormone receptor-positive breast cancer in postmenopausal womenCycle length: Daily and continuous
Early hormone receptor-positive breast cancer in postmenopausal women
Before prescribing: Prescribing notes| Drug | Dose | Route | Schedule |
|---|---|---|---|
| Anastrozole | 1 mg once daily | Oral, with or without food | Every day |
Anastrozole in advanced disease
Postmenopausal women: first line in locally advanced or metastatic breast cancer that is hormone receptor-positive or unknown; or after progression on tamoxifen (ER-negative disease and previous non-response to tamoxifen rarely respond)Cycle length: Daily and continuous
Postmenopausal women: first line in locally advanced or metastatic breast cancer that is hormone receptor-positive or unknown; or after progression on tamoxifen (ER-negative disease and previous non-response to tamoxifen rarely respond)
Before prescribing: Prescribing notes| Drug | Dose | Route | Schedule |
|---|---|---|---|
| Anastrozole | 1 mg once daily | Oral, with or without food | Every day |
Doses follow the trial protocol, guideline or official drug label (source given in the notes of each regimen). Before prescribing, check blood counts, renal and hepatic function, drug interactions and the patient’s condition, and adjust the dose according to the drug label.
Side effects
The most common side effects:
- Hot flushes and joint pain: each in about one in three women [1].
- Tiredness, mood changes, nausea and high blood pressure [1].
- Weaker bones and fractures, and higher cholesterol; this is why bone density and blood fat tests are needed [1].
- Compared with tamoxifen, vaginal bleeding and discharge and blood clots are less common [1].
For physicians
| Adverse event | Anastrozole | Tamoxifen |
|---|---|---|
| Hot flushes | 36% | 41% |
| Joint symptoms (pain, arthritis or arthrosis) | 36% | 29% |
| Fatigue or asthenia | 19% | 18% |
| Mood disturbances | 19% | 18% |
| Nausea and vomiting | 13% | 12% |
| Hypertension | 13% | 11% |
| Osteoporosis | 11% | 7% |
| All fractures | 10% | 7% |
| Hypercholesterolaemia | 9% | 3.5% |
| Vaginal bleeding | 5% | 10% |
| Vaginal discharge | 4% | 13% |
| Carpal tunnel syndrome | 2.5% | 0.7% |
Source: FDA label for Arimidex [1]. The most common reason for stopping treatment in both arms was hot flushes, which was less frequent with anastrozole [1].
Serious side effects
These are less common but serious; tell your team early if you notice any sign of them:
- In women who already have heart disease, heart problems are slightly more common; report chest pain straight away [1].
- Fractures: severe bone pain after a minor knock or fall [1].
- Allergic reaction (rare): swelling of the face, lips, tongue or throat, or blisters and sores on the skin [1].
- Liver inflammation (rare) and harm to an unborn baby [1].
For physicians
Anastrozole has no boxed warning, but the warnings and precautions in its label are [1]:
- Ischaemic cardiovascular events: in women with pre-existing ischaemic heart disease, these events were more frequent with anastrozole (17% vs 10% with tamoxifen); weigh benefit and risk in these patients [1].
- Decreased bone mineral density: in the ATAC bone substudy, lumbar spine and hip density fell with anastrozole and rose with tamoxifen; bone density monitoring is recommended [1]. At 10-year follow-up, fractures were higher only during treatment and did not differ after treatment ended [3].
- Raised cholesterol: 9% vs 3.5% with tamoxifen [1].
- Embryo-fetal toxicity: based on animal studies and mechanism of action, it may harm a fetus [1].
- Rare but serious reactions (fewer than 1 in 10,000 patients): skin lesions, ulcers or blisters; allergic reactions with swelling of the face, lips, tongue or throat; and hepatitis [1]. After marketing, Stevens-Johnson syndrome, anaphylaxis, hypercalcaemia and tendon disorders (including tendon rupture and trigger finger) have also been reported [1].
On the other hand, in ATAC endometrial cancer, vaginal bleeding and discharge, venous thromboembolic events and cerebral ischaemic events were less frequent with anastrozole than with tamoxifen [1].
When should I call my doctor?
Call your doctor or go to the emergency department straight away if you have [1]:
- New or worsening chest pain or shortness of breath
- Swelling of the face, lips, tongue or throat, or difficulty swallowing or breathing
- Lesions, ulcers or blisters on the skin (stop the drug and call straight away)
- Yellowing of the skin or whites of the eyes, pain in the upper right abdomen, or feeling generally unwell
- A possible pregnancy during treatment
Interactions, pregnancy and precautions
- Bone health: your doctor may order bone density scans before and during treatment [1]. The label recommends managing bone health according to guidelines for postmenopausal women with a similar fracture risk; in a post-marketing study, the bisphosphonate risedronate (with calcium and vitamin D) preserved bone density in most patients at risk of fracture [1]. In postmenopausal women with early breast cancer, bone-strengthening drugs (bisphosphonates) have also reduced cancer recurrence in bone and deaths from breast cancer [6].
- Tamoxifen: must not be taken with anastrozole; the combination was no better than tamoxifen alone and lowered anastrozole levels in the blood [1].
- Oestrogen-containing products (hormone replacement therapy, contraceptive pills, and oestrogen creams, rings and pessaries) may reduce the effect of anastrozole [1].
- Pregnancy: women who could become pregnant should have a pregnancy test before starting and use effective contraception during treatment and for at least 3 weeks after the last dose [1].
- Breastfeeding: do not breastfeed during treatment and for 2 weeks after the last dose [1].
- Cholesterol: your doctor may monitor your blood fats with blood tests [1].
Frequently asked questions
Is anastrozole suitable for women who have not yet gone through menopause?
Anastrozole alone does not work before menopause, and its label covers only postmenopausal women [1]. In younger women, if an aromatase inhibitor is needed, it is given with a drug that suppresses the ovaries, as in a large study in which premenopausal women also received ovarian suppression injections [4].
Is anastrozole better than tamoxifen?
In women with early breast cancer after menopause, a review of all the studies showed that this group of drugs lowers the chance of the cancer coming back by about a third compared with tamoxifen, with slightly fewer deaths from breast cancer at 10 years, but more bone fractures [7]. In a large study, serious side effects were less common with anastrozole than with tamoxifen [3].
Is joint pain normal with anastrozole?
Yes, joint pain is common; about one in 3 women taking anastrozole has it, a little more than with tamoxifen [1]. If the pain bothers you, talk to your doctor and do not stop the drug on your own.
Can I use hormone creams or tablets for hot flushes or vaginal dryness?
Oestrogen-containing products, including hormone replacement therapy and oestrogen creams, rings or pessaries, may reduce the effect of anastrozole [1]. Talk to your doctor before using any hormonal or herbal product.
Is anastrozole also used to prevent breast cancer?
Scientific evidence
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Key studies
4 itemsBreast Cancer, Version 4.2026, NCCN Clinical Practice Guidelines In Oncology
Pan-Asian adapted ESMO Clinical Practice Guidelines for the diagnosis, treatment and follow-up of patients with early breast cancer
Adjuvant Abemaciclib Plus Endocrine Therapy for Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-Negative, High-Risk Early Breast Cancer: Results From a Preplanned monarchE Overall Survival Interim Analysis, Including 5-Year Efficacy Outcomes
Ribociclib plus Endocrine Therapy in Early Breast Cancer
Related cancer types
References
7 referencesShow all references, 6 with a study cardClose the reference list
Numbers in the text, such as [1], refer to these sources. Sources with a “Study card” button are summarised in the site’s evidence library.
- 1
- 2
- 3
- 4
- 5
- 6Adjuvant bisphosphonate treatment in early breast cancer: meta-analyses of individual patient data from randomised trials
- 7Aromatase inhibitors versus tamoxifen in early breast cancer: patient-level meta-analysis of the randomised trials
This page is based on reputable scientific sources and was last updated on 27 October 2026. The content of this website is for educational and informational purposes only and does not replace consultation, diagnosis or treatment by a specialist physician. Consult your treating physician about any treatment decision.
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