Anastrozole Drug Interactions: What to Avoid and What to Tell Your Doctor

Anastrozole is generally well-tolerated and doesn't have many serious drug-to-drug interactions — but the interactions it does have are critical to know about. Taking anastrozole with certain medications can make your breast cancer treatment significantly less effective or cause serious harm. This guide covers everything you need to know about anastrozole interactions in 2026.

The Two Contraindicated Combinations: Never Take These With Anastrozole

There are two categories of medications that are absolutely contraindicated (should never be used) with anastrozole:

1. Tamoxifen

Tamoxifen is another breast cancer hormone therapy, but it works differently from anastrozole. When tamoxifen and anastrozole are taken together, tamoxifen reduces anastrozole's blood levels by approximately 27% — making anastrozole significantly less effective at suppressing estrogen. The combination also does not provide better cancer control than either drug alone, based on the landmark ATAC trial. Tamoxifen and anastrozole should not be administered together under any circumstances.

2. Estrogen-containing products

Anastrozole works by lowering estrogen levels. Taking any estrogen-containing product directly counteracts this effect — essentially undoing your cancer treatment. The following estrogen products are contraindicated with anastrozole:

Hormone replacement therapy (HRT) tablets, patches, rings, or injections

Estrogen creams, gels, or vaginal suppositories

Estrogen-containing birth control pills, implants, or injections

Estradiol or other estrogen formulations in any form

This includes vaginal estrogen products like estradiol vaginal cream, Vagifem tablets, and vaginal estrogen rings. Even low-dose vaginal estrogen can be absorbed systemically and may reduce anastrozole's effectiveness. Discuss non-hormonal options for vaginal dryness with your oncologist.

Interactions That Require Caution

These interactions are less critical than the two above, but should still be discussed with your doctor:

Warfarin (blood thinner): Anastrozole does not significantly affect warfarin's blood-thinning effects in studies, but some individual variation occurs. If you take warfarin, your INR should be monitored when starting or stopping anastrozole.

CYP3A4 substrates: Anastrozole is a mild inhibitor of certain liver enzymes (CYP3A4 and others). At the standard 1 mg dose, this inhibition is unlikely to cause clinically significant interactions with other drugs, but theoretically could slightly increase levels of some medications. Your pharmacist can check for specific interactions.

Etrasimod and other immunomodulating agents: Because anastrozole has mild immunosuppressive effects as an antineoplastic agent, combining it with other immunosuppressants may compound immune effects. Discuss any immunosuppressant therapy with your oncologist.

Live vaccines: Due to mild immunosuppressive effects, live vaccines may have reduced efficacy or increased risk of infection when administered during anastrozole therapy. Discuss vaccine timing with your oncologist.

What About Herbal Supplements?

Several herbal supplements can potentially interact with anastrozole:

Phytoestrogens (soy, red clover, flaxseed): Plant compounds that mimic estrogen's effects. In theory, high doses could reduce anastrozole's effectiveness. While modest dietary consumption of soy is generally considered acceptable, high-dose soy or phytoestrogen supplements should be discussed with your oncologist.

St. John's Wort: A CYP3A4 inducer that can reduce plasma levels of many drugs. May reduce anastrozole levels, potentially reducing its effectiveness. Avoid St. John's Wort while taking anastrozole.

Black cohosh: Often used for menopause symptoms; some concern about estrogen-like effects, though evidence is mixed. Discuss with your oncologist before using.

What About Alcohol?

There are no known clinically significant interactions between anastrozole and alcohol. However, alcohol consumption increases the risk of breast cancer recurrence and is associated with increased cancer risk generally. Your oncologist may advise limiting or eliminating alcohol during treatment for this reason, not specifically because of a drug interaction.

What to Tell Your Doctor Before Starting Anastrozole

Before starting anastrozole, tell your doctor and pharmacist about ALL of the following:

Every prescription medication you take (especially tamoxifen, estrogen products, warfarin)

All over-the-counter medications (NSAIDs, supplements, vitamins)

All herbal supplements (especially St. John's Wort, black cohosh, soy supplements)

Any liver disease, kidney disease, or heart disease history

Related Guides

Read our full guide on anastrozole side effects and review our complete overview of what anastrozole is for additional context.