What Vitamins Should Not Be Taken With Anastrozole?

October 7, 2025 •

4 min read

While specific vitamin-drug interactions are not a primary concern with anastrozole, a 2024 review highlights the importance of avoiding concentrated herbal supplements that contain phytoestrogens, as these can counteract the medication's therapeutic effect. This guide details what vitamins should not be taken with anastrozole and provides crucial information on potentially harmful supplements.

Quick Summary

This article clarifies that while vitamins generally do not interact with anastrozole, supplements containing phytoestrogens or certain herbs must be avoided to prevent a reduction in the drug's effectiveness.

Key Points

Specific Vitamin Interactions Are Not the Main Concern: Anastrozole does not typically interact with standard vitamins like B12 or C; multivitamins are generally considered safe.
Avoid Phytoestrogen-Containing Supplements: Concentrated supplements containing plant-based estrogens (phytoestrogens) can interfere with anastrozole's anti-estrogen function and should be avoided.
Steer Clear of Specific Herbs: Certain herbal supplements like St. John's Wort and concentrated ginseng can negatively interact with anastrozole and should not be taken.
Vitamin D and Calcium Are Often Recommended: Anastrozole can increase the risk of bone density loss, making supplementation with vitamin D and calcium a common and often necessary component of treatment.
Inform Your Healthcare Team of All Supplements: Due to the potential for unlisted interactions and the varying concentrations of active compounds, it is crucial to tell your doctor about every supplement you take.
Be Cautious with Grapefruit and Concentrated Extracts: Some experts advise caution with grapefruit due to its impact on drug metabolism enzymes, and high-dose botanical extracts like green tea should be discussed with an oncologist.

What Vitamins Should Not Be Taken With Anastrozole? Understanding the Nuance

When a patient is prescribed anastrozole (brand name Arimidex), a type of aromatase inhibitor used to treat hormone-receptor-positive breast cancer in postmenopausal women, they may have questions about supplement safety. Anastrozole works by blocking the aromatase enzyme, which is responsible for producing estrogen in the body, thereby starving cancer cells of the hormone they need to grow. Therefore, any substance that increases estrogen levels or mimics its effects can interfere with this critical mechanism. The key takeaway is not about traditional vitamins, but rather about concentrated supplements derived from plants and herbs.

The Critical Distinction: Vitamins vs. Supplements

It is important to differentiate between standard vitamins and herbal or dietary supplements. Pure vitamins, such as vitamins B12, C, or most types found in a typical multivitamin, do not have significant or documented interactions with anastrozole. In fact, some, like vitamin D, are often recommended. The danger lies in herbal products and other supplements that contain compounds, such as phytoestrogens, that can counteract the anti-estrogen action of anastrozole.

Supplements to Avoid While on Anastrozole

Phytoestrogen-Containing Supplements

Phytoestrogens are plant-based compounds that have a weak estrogen-like effect in the body. When consumed in high concentrations via supplements, they can potentially reduce the effectiveness of anastrozole. Examples of supplements containing phytoestrogens that should be avoided include:

Soy protein supplements: Concentrated soy extracts contain isoflavones, a type of phytoestrogen.
Black cohosh: Often used for menopausal symptoms, but can oppose anastrozole's action.
Red clover: Another source of isoflavones used for menopause relief.
Dong Quai: An herb traditionally used to treat gynecological conditions.
Flaxseed supplements: While ground flaxseed in food is generally safe, concentrated flaxseed supplements should be approached with caution due to high phytoestrogen content.
Evening primrose oil: An herb with phytoestrogenic effects.
Licorice root: Can have phytoestrogenic properties.
Resveratrol: While present in grapes, concentrated supplements may pose a risk.

Other Herbal Supplements with Known Interactions

Beyond phytoestrogens, other herbal supplements can affect drug metabolism and should be avoided or used with extreme caution. St. John's Wort, a popular supplement for depression, is known to interact with and decrease the effectiveness of anastrozole and other medications. Ginseng is another herbal remedy that has been identified as a potential source of interaction.

Potential Effects of Other Concentrated Extracts

Some concentrated botanical extracts, such as green tea extract, have been noted for their potential to interact with certain therapies. While green tea itself is generally safe, the high concentration in extract form has raised concerns about potential hepatotoxicity and should be discussed with an oncologist. Similarly, grapefruit and its juice have long been known to interfere with enzymes that metabolize many drugs, and while not confirmed for anastrozole specifically, many experts recommend caution.

Recommended Supplements During Anastrozole Therapy

Vitamin D and Calcium for Bone Health

Aromatase inhibitors like anastrozole can increase the risk of osteoporosis and bone fractures. For this reason, supplementation with vitamin D and calcium is often considered standard care and is recommended by multiple oncology organizations. Vitamin D helps the body absorb calcium, which is crucial for maintaining bone density. The dosage and specific supplements should always be determined and monitored by a healthcare professional.

A Comparative Look at Supplement Safety


Supplement Category	Safety with Anastrozole	Rationale	Examples	Recommendation
Standard Vitamins	Generally Safe	Typically do not interfere with drug metabolism.	Vitamin B12, Vitamin C, Multivitamins (check ingredients).	Safe to take; discuss with doctor.
Vitamin D & Calcium	Recommended	Counteracts bone density loss, a common side effect of anastrozole.	Calcium carbonate, Vitamin D3.	Often prescribed by your oncologist.
Phytoestrogen Supplements	Avoid	Can mimic estrogen, reducing anastrozole's effectiveness.	Soy protein, Red clover, Dong Quai.	Do not take without discussing with a doctor.
St. John's Wort	Avoid	Known to interfere with drug metabolism.	St. John's Wort supplements.	Do not take; consult doctor for alternatives.
Ginseng	Caution	Possible interactions with aromatase inhibitors.	Panax ginseng, other ginseng forms.	Discuss with oncologist; generally best to avoid concentrated forms.

Conclusion: Always Consult Your Healthcare Team

While the answer to "what vitamins should not be taken with anastrozole?" is that most standard vitamins are safe, the real risk lies in concentrated herbal and botanical supplements. These products are not regulated as strictly as pharmaceuticals and can contain active compounds that interfere with anastrozole's mechanism of action. Therefore, it is critical to inform your entire healthcare team—including your oncologist and pharmacist—about every supplement, herb, and over-the-counter product you use. For managing bone health, supplementing with vitamin D and calcium is often recommended, but even these should be taken under medical guidance. By maintaining clear communication with your doctors, you can ensure your safety and maximize the effectiveness of your cancer treatment. For further guidance, patients can consult the National Cancer Institute's resources on cancer treatment and supportive care: National Cancer Institute.

The Critical Role of Your Healthcare Team

Your oncologist and pharmacist are the best resources for personalized advice. They can review your complete medication and supplement list to identify any potential risks. Due to individual patient variations, what may be safe for one person may not be for another. This includes supplements sometimes used to manage side effects, such as magnesium glycinate for musculoskeletal pain, which should be discussed with your care team first. Monitoring and open communication are the best strategies for managing your health while on anastrozole.

Frequently Asked Questions

Anastrozole (brand name Arimidex) is a medication used to treat hormone-receptor-positive breast cancer in postmenopausal women. It is an aromatase inhibitor that works by blocking the enzyme aromatase, which reduces the amount of estrogen in the body and prevents cancer cells from growing.

Yes, standard multivitamins are generally considered safe with anastrozole and do not cause significant interactions. However, you should always check the ingredients list to ensure the multivitamin does not contain any potentially interacting herbal extracts or high concentrations of phytoestrogens.

Yes, vitamin D is not only safe but is often recommended alongside calcium for women taking anastrozole. Anastrozole can increase the risk of osteoporosis, and vitamin D and calcium supplementation helps to maintain bone health and density.

Phytoestrogens are plant compounds that can mimic estrogen in the body, even if weakly. Taking concentrated phytoestrogen supplements, like those from soy or red clover, can counteract the estrogen-lowering effects of anastrozole, potentially making the cancer treatment less effective.

St. John's Wort is a specific herbal supplement that should be avoided entirely while taking anastrozole, as it is known to significantly affect the metabolism of certain medications, including aromatase inhibitors.

Eating foods that naturally contain low levels of phytoestrogens, such as flaxseed or soy in a regular diet, is generally not a concern. The potential for interaction is primarily with concentrated extracts found in supplements.

The most important rule is to always inform your oncologist and pharmacist about every supplement, herb, and medication you are taking, regardless of how minor it may seem. They can provide guidance to ensure your safety and the maximum effectiveness of your treatment.