Several categories of supplements can interfere with anastrozole, primarily those that raise estrogen levels, provide raw materials your body converts into estrogen, or alter how the drug is metabolized. Since anastrozole works by blocking your body’s ability to produce estrogen, anything that adds estrogen back into the equation can undermine the entire point of treatment.
Why These Interactions Matter
Anastrozole belongs to a class of drugs called aromatase inhibitors. It works by shutting down aromatase, the enzyme responsible for converting androgens into estrogen in your body. For postmenopausal women with estrogen-dependent breast cancer, this is critical: cutting off the estrogen supply starves tumors that rely on it to grow. Supplements that reintroduce estrogen, stimulate its production, or boost aromatase activity can directly counteract this mechanism. The concern isn’t theoretical. Some of these supplements contain compounds with measurable estrogenic effects, and in concentrated supplement form, those effects can be significant.
Phytoestrogen Supplements
Phytoestrogens are plant compounds that mimic estrogen in your body. In the small amounts found in everyday foods, they’re generally not a problem. In concentrated supplement form, the dose is high enough to potentially interfere with anastrozole. The following phytoestrogen-containing supplements are flagged as problematic:
- Red clover, one of the most concentrated sources of plant estrogens
- Soy isoflavone supplements (genistein, daidzein, or equol in pill form)
- Black cohosh, commonly marketed for hot flash relief
- Dong quai, a traditional Chinese herb for menstrual and menopausal symptoms
- Licorice root
- Flaxseed supplements (concentrated lignans)
- Fennel and anise/star anise
- Alfalfa and kudzu
- Wild yam
This is a particular frustration for many women because several of these herbs are sold specifically to ease menopause-like side effects, which are extremely common on anastrozole. Hot flashes, joint pain, and sleep disruption drive people to look for relief, and the supplement aisle is full of products that could work against their treatment.
The Soy Question: Supplements vs. Food
Soy is one of the most confusing topics for women on anastrozole, and the answer depends on the form. Soy isoflavone supplements, the kind sold in capsules as a menopause remedy, remain controversial and are generally advised against. However, dietary soy (tofu, edamame, soy milk) tells a different story. A large study of postmenopausal women on anastrozole found that those with the highest dietary soy intake actually had an 18.7% lower recurrence rate compared to those with the lowest intake. The protective association was statistically significant and specific to women on anastrozole with hormone-receptor-positive disease. So eating moderate amounts of soy foods appears safe and possibly beneficial, while taking concentrated soy isoflavone pills is a different risk calculation entirely.
Hormonal Precursors: DHEA, Pregnenolone, and Androstenedione
Your body makes estrogen through a chain of conversions that starts with cholesterol. Supplements that provide building blocks along that chain give your body extra raw material to produce estrogen, even while anastrozole is trying to block the final step. DHEA is a particular concern. In postmenopausal women, circulating DHEA from the adrenal glands is the primary source of androgens that get converted into estrogen at peripheral sites like breast tissue. The breast itself expresses all the enzymes needed to convert DHEA into estrogen, including aromatase. Supplementing with DHEA essentially hands your body more fuel for estrogen production.
Pregnenolone and androstenedione work the same way, sitting upstream in the hormone pathway. Deer velvet, which contains androstenediol and other hormone precursors, falls into this category too. Any supplement marketed as a “hormone balancer” or “adrenal support” that contains these compounds should be avoided.
Herbal Supplements That Boost Estrogen or Aromatase
Beyond phytoestrogens and hormone precursors, a number of popular herbal supplements either stimulate estrogen production directly or increase aromatase activity, which is the exact enzyme anastrozole is designed to suppress:
- Ginseng (including Panax ginseng)
- Maca root
- Milk thistle
- Turmeric/curcumin supplements
- Resveratrol
- Quercetin supplements
- St. John’s wort, which also alters how your liver processes many drugs
- Reishi mushroom
- Astragalus (huang qi)
- Goldenseal
- Chasteberry
- Horny goat weed
- Sage supplements and sage essential oil
- Propolis and royal jelly
- Saw palmetto
- Valerian
- Guggul
- Noni juice
- DIM (diindolylmethane)
Several of these are surprising. Turmeric and milk thistle, for example, are widely considered “healthy” supplements, and resveratrol is marketed as an antioxidant powerhouse. But in concentrated supplement doses, each has shown potential to increase estrogenic activity or interfere with aromatase inhibitor therapy. Again, the amounts found naturally in food (turmeric in cooking, grapes containing trace resveratrol) are a different matter than high-dose capsules.
Copper and Multivitamins Containing Copper
Copper supplements and multivitamins with added copper have been flagged for potentially increasing aromatase activity. If you take a daily multivitamin, check the label for copper content. Many standard formulations include it. A multivitamin without copper, or one specifically formulated for breast cancer patients, may be a better choice.
What About Bioidentical Hormones and HRT?
Combined hormone replacement therapy, including products marketed as “bioidentical” hormones, directly introduces estrogen into your body. This completely undermines what anastrozole is doing. Even topical estrogen creams and vaginal estrogen products should be discussed with your oncologist before use. The word “bioidentical” can make these products sound gentler or more natural, but your body processes them as estrogen regardless of their source.
Supplements That Are Safe and Encouraged
Anastrozole accelerates bone density loss because estrogen plays a key role in maintaining bone strength. Calcium and vitamin D are not only safe but recommended. The Institute of Medicine recommends 1,000 mg of calcium daily for women ages 19 to 50 and 1,200 mg for women over 51, along with 600 IU of vitamin D daily for women under 70 and 800 IU for those over 70. That said, research on women undergoing breast cancer therapy suggests that even these standard doses (500 to 1,500 mg calcium, 200 to 1,000 IU vitamin D) were often inadequate to fully prevent bone mineral density loss. Your oncologist may recommend higher vitamin D doses based on your blood levels.
Omega-3 fatty acids from fish oil are generally considered safe during aromatase inhibitor therapy and may help with the joint pain that many women experience. Vitamin B12, folate, and magnesium don’t appear to interact with anastrozole, though you should still mention them to your care team.
The Bigger Picture on Supplements
Most supplements have never been formally studied for interactions with aromatase inhibitors. The absence of a supplement from warning lists doesn’t mean it’s been proven safe. It often just means no one has tested it yet. The supplements flagged above are the ones where evidence exists for concern, but the overall landscape is one of limited data. If you’re considering any supplement not mentioned here, bring the specific product (with its ingredient list) to your oncologist or pharmacist. Combination products and “women’s health” blends frequently contain small amounts of multiple problematic herbs, and the names on the label don’t always make the ingredients obvious.