What Medications Should You Not Take with Anastrozole?

October 10, 2025 •

4 min read

According to the U.S. Food and Drug Administration (FDA), anastrozole (marketed under the brand name Arimidex) is contraindicated with drugs containing estrogen. Understanding what medications should you not take with anastrozole is critical for ensuring the drug's effectiveness and preventing serious health complications, particularly for postmenopausal women with hormone receptor-positive breast cancer.

Quick Summary

Anastrozole has significant drug interactions, primarily with estrogen-containing products and tamoxifen, which can negate its therapeutic effects. Patients must inform their healthcare providers about all medications and supplements, including over-the-counter and herbal products, to avoid harmful combinations.

Key Points

Strictly Avoid Estrogen-Containing Products: Do not take Hormone Replacement Therapy (HRT), estrogen-based birth control pills, or topical estrogen creams, as they directly counteract anastrozole's mechanism of action.
Do Not Combine with Tamoxifen: Anastrozole and tamoxifen should not be taken concurrently, as tamoxifen can decrease anastrozole levels in the body, reducing its effectiveness.
Exercise Caution with Herbal Supplements: Avoid herbal remedies and supplements that contain phytoestrogens, such as black cohosh, soy isoflavones, and concentrated flaxseed, as they can interfere with anastrozole's action.
Discuss All Medications with Your Doctor: Always inform your healthcare provider about all prescription, over-the-counter (OTC) drugs, vitamins, and supplements to prevent unforeseen drug interactions.
Consult Before Getting Vaccinated: Some live vaccines may have a reduced effect when taken with anastrozole; discuss the appropriate timing with your doctor.
Manage Bone Health Carefully: Anastrozole can decrease bone mineral density, and while not an interaction, this requires careful monitoring and often necessitates calcium and Vitamin D supplementation.
Monitor Cholesterol Levels: Anastrozole can increase cholesterol levels, and patients with pre-existing hypercholesterolemia should be monitored closely.
Avoid during Pregnancy or Breastfeeding: Anastrozole is contraindicated in pregnant women and those who are breastfeeding due to potential harm to the fetus or infant.

Anastrozole is a potent non-steroidal aromatase inhibitor, a class of drugs that works by blocking the enzyme aromatase, which is responsible for converting androgens into estrogen in the body. This reduction in estrogen is crucial for treating hormone receptor-positive breast cancer in postmenopausal women. However, certain medications and supplements can interfere with this process, rendering anastrozole ineffective or increasing the risk of adverse effects. Therefore, it is essential to have a comprehensive understanding of what to avoid while undergoing anastrozole therapy.

Estrogen-Containing Medications

The most significant and obvious group of medications to avoid with anastrozole are those containing estrogen. Since anastrozole's primary function is to suppress estrogen production, introducing external sources of estrogen directly counteracts its mechanism of action. This can render the breast cancer treatment ineffective and increase the risk of recurrence. This group includes, but is not limited to:

Hormone Replacement Therapy (HRT): Medications used to alleviate menopause symptoms, such as Premarin, Estrace, and Estraderm, should not be taken with anastrozole.
Estrogen-based Contraceptives: Birth control pills, patches, or rings that contain estrogen are contraindicated with anastrozole.
Topical Estrogen Creams: Any estrogen-based topical applications should be avoided as they can be absorbed systemically.
Other estrogen derivatives: Compounds like estradiol, conjugated estrogens, and estropipate are also on the list of drugs to avoid.

Tamoxifen

Another critical drug interaction to be aware of is with tamoxifen, a different type of hormonal therapy for breast cancer. The combination of anastrozole and tamoxifen is generally contraindicated, as studies have shown that tamoxifen can decrease the plasma concentration of anastrozole, making it less effective. Some studies in the early 2000s explored the combination, but it was discontinued due to a lack of added efficacy and conflicting results. The two therapies work through different mechanisms, and combining them does not offer a greater benefit than using anastrozole alone.

Herbal Remedies and Supplements

It is a common misconception that because herbal remedies and supplements are considered "natural," they are safe to take with prescription medication. However, many herbal products contain phytoestrogens or ingredients that can mimic or increase estrogen levels in the body. These can directly interfere with anastrozole's function. Examples of such supplements include:

Soy-based products and isoflavones: These contain compounds that can act as phytoestrogens.
Black cohosh: Often used for menopausal symptoms, but can interfere with anastrozole.
Red clover: Contains isoflavones and should be avoided.
Flaxseed: Also contains phytoestrogens and should be consumed in moderation, or avoided in concentrated supplement forms.

Always discuss any supplements or herbal teas you are taking with your doctor or pharmacist to ensure there is no interaction with your anastrozole treatment.

Key Considerations for Drug Interactions with Anastrozole

Beyond direct counteractions with estrogen, other types of drugs can have complex interactions with anastrozole through metabolic pathways. While fewer in number, these warrant careful consideration and monitoring by a healthcare professional. Some specific examples include:

Certain Antineoplastic Agents: Some chemotherapy drugs, like palifermin, should be avoided within 24 hours of anastrozole administration to prevent increased severity and duration of side effects like oral mucositis.
Some Vaccines: Live vaccines, such as measles, mumps, and rubella (MMR) or yellow fever, may have a reduced immune response when given with anastrozole due to its potential immunosuppressive effects.
Warfarin: Anastrozole has been reported to interfere with warfarin in some instances. Patients on blood thinners should be closely monitored.
Other Immunosuppressive Therapies: Combining anastrozole with other immunosuppressive agents like etrasimod can increase the risk of infection and requires careful consideration.

Comparison of Key Interactions with Anastrozole

To better illustrate the potential conflicts, the following table summarizes the primary categories of medications that interact with anastrozole and the nature of the interaction.


Medication Class	Example Medications	Nature of Interaction	Impact on Anastrozole	Action to Take
Estrogen-Containing Drugs	HRT (Premarin), Estrogen-based Birth Control	Pharmacodynamic antagonism	Negates the estrogen-suppressing effect	Strictly Avoid
Tamoxifen	Nolvadex, Soltamox	Decreases anastrozole plasma concentration	Reduces anastrozole's therapeutic effectiveness	Strictly Avoid Concurrent Use
Herbal/Phytoestrogen Supplements	Black Cohosh, Soy Isoflavones	Introduces estrogen-mimicking compounds	Counteracts the estrogen suppression effect	Avoid Concentrated Doses
Certain Live Vaccines	MMR, Yellow Fever	Potential additive immunosuppressive effects	Reduces immune response to the vaccine	Use Caution/Discuss with Doctor

Conclusion

For patients on anastrozole therapy, the most crucial interactions to avoid are those with any form of estrogen, including hormonal contraceptives and Hormone Replacement Therapy (HRT), and the breast cancer medication tamoxifen. Patients should also be cautious with herbal supplements and certain over-the-counter products that may contain estrogen-like compounds. Always maintain open and transparent communication with your healthcare team, including your oncologist and pharmacist, about all medications and supplements you are taking. This proactive approach is the best way to ensure the safety and efficacy of your anastrozole treatment. Never start, stop, or change any medication without consulting your doctor first.

Disclaimer: This article is for informational purposes only and does not constitute medical advice. Consult a healthcare professional for diagnosis and treatment.

Frequently Asked Questions

No, you should not take hormone replacement therapy (HRT) or any other estrogen-containing medications with anastrozole. The purpose of anastrozole is to lower estrogen levels, and HRT would counteract this effect, making the cancer treatment ineffective.

No, co-administration of tamoxifen and anastrozole is not recommended. Studies have shown that tamoxifen can significantly decrease the concentration of anastrozole in the blood, reducing its therapeutic effect. The combination has not shown any additional benefit over single-drug therapy.

You should avoid any herbal remedies or supplements that contain phytoestrogens or are marketed for menopausal symptoms. Common examples include black cohosh, red clover, and concentrated soy or flaxseed supplements, as they can mimic estrogen and interfere with anastrozole.

While there are few direct interactions with common OTC medications, you must inform your doctor about all of them. Certain OTC products, like those containing pseudoephedrine or other decongestants, should be used with caution, particularly if you have pre-existing heart conditions.

Yes, anastrozole can decrease bone mineral density over time due to its estrogen-lowering effects, which can increase the risk of osteoporosis and fractures. Your doctor may recommend specific calcium and Vitamin D supplements, and sometimes bisphosphonates, to help mitigate this effect.

You should discuss any vaccine with your healthcare provider. Some live vaccines may be less effective or carry a risk of additive immunosuppressive effects, so caution is advised, and your doctor can determine the best timing.

If you accidentally take anastrozole with a contraindicated medication, such as tamoxifen or an estrogen product, you should contact your doctor immediately. Do not panic, but seek professional medical advice to determine the next steps and whether any monitoring is necessary.