Many beneficial micronutrients have been found to be associated with reduced breast cancer risk. When such compounds are consumed in food, they are likely to have subtle chemopreventive effects, whereas the same micronutrients administered at high doses are more likely to have pharmacological effects, with mostly unknown results. Therefore, it is best to obtain beneficial compounds by consuming food, if possible. Generally speaking, the best use of supplements is to make up for deficiencies that are difficult to correct through diet.
There is very little interest among researchers in the topic of adverse effects of chemopreventive compounds; few studies have been published for many of them. Often a micronutrient's beneficial effects may be established and described in detail without considering how it might interact with breast cancer treatment. We provide information in this web page on the possible adverse effects of taking some supplements, including vitamins. Given the sparseness of research, the information is necessarily incomplete.
Click on any supplement name below to obtain more information concerning it.

Supplements with warnings for breast cancer patients and survivors

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Amygdalin
Can be toxic, especially if taken orally or with vitamin C. Also known as Laetrile or vitamin B17.
Apigenin
Can act both as an estrogen and as an anti-estrogen depending on the circumstances and has the potential to stimulate the growth of ER+ breast cancer. Apigenin supplementation has been shown to reduce the effectiveness of chemotherapy and radiotherapy under some conditions.
Apricot kernels
Can result in cyanide toxicity. Laetrile, a synthetic form of amygdalin (found in apricot kernels), can also be toxic, especially if taken orally or with vitamin C. Laetrile is also known as vitamin B17.
Astragalus
A herb that can act both as an estrogen and as an anti-estrogen depending on dosage and has the potential to stimulate growth of ER+ breast cancer. One study showed that astragalus promoted proliferation of ER+ breast cancer via estrogen receptor and ERK1/2 activation both in breast cancer cells and an animal model of breast cancer. Also known as huang qi.
Bitter almond oil
Can cause cyanide poisoning and should not be ingested or used on the skin unless prussic acid-free.
Bitter apple
Citrullus colocynthis, closely related to watermelon, but with a much higher fraction of cucurbitacins, which makes it toxic and means it must be used very cautiously. Also known as colocynth and bitter cucumber.
Bitter cucumber
Citrullus colocynthis, closely related to watermelon, but with a much higher fraction of cucurbitacins, which makes it toxic and means it must be used very cautiously. Also known as colocynth and bitter apple.
Bitter gourd
Momordica charantia, high dosages can cause low blood sugar and chronic use can cause kidney damage. Use only under careful supervision and monitoring. Also known as bitter melon.
Bitter melon
Momordica charantia, high dosages can cause low blood sugar and chronic use can cause kidney damage. Use only under careful supervision and monitoring. Also known as bitter gourd.
Black cohosh
Black cohosh has been shown to interfere with the effectiveness of anthracycline (Adriamycin, epirubicin) chemotherapy. Can also reduce the efficacy of tamoxifen by inhibiting formation of active tamoxifen metabolites.
Blue-green algae
Such supplements sometimes contain concentrated amounts of heavy metals, toxic compounds such as microcystins, and/or inedible microalgaes, depending on where the blue-green algae is grown.
Bromelain
Bromelain and pineapple juice (which contains bromelain) have demonstrated potent inhibition of P450 2C9 (CYP2C9) drug metabolizing activity, suggesting that they should be avoided during tamoxifen treatment.
Caffeine
Caffeine can interfere with the treatment effects of Adriamycin and other intercalating chemotherapy drugs by forming molecular complexes that block the drug's actions. Caffeine can also reduce the cytotoxic effects of Taxol and other taxane chemotherapy drugs via down-regulation of α-tubulin acetylation.
Capsaicin
High intake of capsaicin has been linked to increased risk of gastric cancer independent of H. pylori infection.
Chamomile
Taking chamomile supplements can interfere with the absorption of non-heme iron (found in non-animal sources such as dry beans and iron supplements).
Chinese red yeast rice
Contains a mixture of monacolins, one of which is essentially identical to lovastatin, a statin used for lowering cholesterol. Therefore, Chinese red yeast rice has the same potential side effects as lovastatin. The monacolin content of commercially available Chinese red yeast rice supplements can vary greatly and may be lower than expected. Also known as red mold rice.
Citrullus colocynthis
Closely related to watermelon, but with a much higher fraction of cucurbitacins, which makes it toxic and means it must be used very cautiously. Also known as colocynth, bitter apple, and bitter cucumber.
CLA
Although study findings concerning breast cancer are inconsistent, the t10,c12-CLA normally found in CLA supplements has been found to promote mammary tumor development in experiments using a mouse model of HER2+ breast cancer.
Colchicum autumnale
Also known as autumn crocus, meadow saffron, or wild saffron, Colchicum autumnale is an unrelated and poisonous plant that should not be confused with saffron and is to be avoided.
Colocynth
Citrullus colocynthis, closely related to watermelon, but with a much higher fraction of cucurbitacins, which makes it toxic and means it must be used very cautiously. Also known as bitter apple and bitter cucumber.
Copper
While those with low iron stores are at risk of anemia if they do not have adequate copper intake, copper deficiency is rare and copper supplements should not be taken by those who do not need them. Copper has been shown to promote angiogenesis, facilitate breast cancer cell migration and invasion, and help cancer cells evade the immune system.
Cucurbitacins
Cucurbitacins can be toxic when ingested in large amounts or concentrated form.
Curcumin
Curcumin and turmeric supplements should be avoided by those taking tamoxifen since they can interfere with the metabolism of tamoxifen and reduce its treatment effects (see Tamoxifen and turmeric or curcumin supplements).
Daidzein
Taking soy phytoestrogen supplements during tamoxifen or aromatase inhibitor treatment may promote tumor growth. Daidzein, equol or genistein supplements might also promote breast cancer growth and proliferation under some other conditions, including in women with benign breast disease and in breast cancer survivors.
Danshen
Significant herb-drug interaction have been observed between danshen (the dried root and rhizome of Salvia miltiorrhiza) and warfarin, aspirin, and digoxin, among others. CYP induction by danshen suggests that it should be avoided during tamoxifen treatment. Salvia miltiorrhiza has also been reported to have potential angiogenic properties. Also known as red sage root or salvia root.
EGCG
EGCG and green tea supplements and herbal mixtures containing green tea in concentrated form have been shown to have the potential to cause liver damage. They have also been shown to interfere with the effectiveness of anthracycline (Adriamycin, epirubicin) chemotherapy.
Equol
Taking soy phytoestrogen supplements during tamoxifen or aromatase inhibitor treatment may promote tumor growth. Genistein, daidzein or equol supplements might also promote breast cancer growth and proliferation under some other conditions, including in women with benign breast disease and in breast cancer survivors.
Essiac tea
Has been shown to stimulate both ER+ and ER- breast cancer cell growth.
Fennel seed
Reports that fennel is being used to increase breast milk production are concerning because they suggest that fennel acts directly on breast tissue. Fennel has been shown to have estrogenic properties.
Fish oil
Could neutralize the treatment effects of platinum-based chemotherapy drugs such as cisplatin.
Flaxseed
Can have high cadmium content unless specifically labeled low-cadmium (organic does not necessarily mean low-cadmium).
Foeniculum vulgare
Reports that fennel (Foeniculum vulgare) is being used to increase breast milk production are concerning because they suggest that fennel acts directly on breast tissue. Fennel has been shown to have estrogenic properties.
Folic acid
While dietary folate is associated with reduced risk of breast cancer and its recurrence, folic acid (the synthetic form of folate found in supplements and fortified foods) has been shown to reduce natural killer cell cytotoxicity and to promote existing tumor growth in mouse models of breast cancer.
Fucoidan
Fucoidan supplementation may result in increased blood thinning, depending on the formulation.
Ganoderma lucidum
Reishi (Ganoderma lucidum) extract has been found to have estrogenic effects, significantly increasing uterine weight in young rats in one study and promoting ER+/PR+ cell proliferation in another. Reishi has also been shown to reduce the effectiveness of Adriamycin in ER+/PR+ breast cancer cells. Reishi probably should be avoided by those with ER+ breast cancer, with the exception of patients with inflammatory breast cancer (IBC); it is one of the few foods shown to be effective in inhibiting IBC progression.
Garlic
Garlic supplements have been shown to interfere with the activities of taxane (Taxol, Taxotere) and platinum-based (cisplatin) chemotherapy drugs.
Genistein
Taking soy phytoestrogen supplements during tamoxifen or aromatase inhibitor treatment may promote tumor growth. Genistein, daidzein or equol supplements might also promote breast cancer growth and proliferation under some other conditions, including in women with benign breast disease and in breast cancer survivors.
Goldenseal
Goldenseal can interfere with the effectiveness of taxanes (Taxol, Taxotere), anthracyclines (Adriamycin, epirubicin), cisplatin, tamoxifen, aromatase inhibitors and many other drugs. Toxic at high doses.
Hesperidin
Has the potential to interfere with taxane (Taxol, Taxotere) and cyclophosphamide chemotherapy. Cyclophosphamide is frequently used in combination with anthracyclines (Adriamycin, epirubicin) and/or taxanes to treat breast cancer.
Huang Qi
A herb that can act both as an estrogen and as an anti-estrogen depending on dosage and has the potential to stimulate growth of ER+ breast cancer. One study showed that it promoted proliferation of ER+ breast cancer via estrogen receptor and ERK1/2 activation both in breast cancer cells and an animal model of breast cancer. Also known as astragalus.
Iodine
High intake may result in elevated blood pressure and blood sugar.
Iron
While iron deficiency should be avoided, high levels of iron or ferritin (a molecule that stores iron) have been shown to reduce the cytotoxic treatment effects of Adriamycin, Taxol and cisplatin. Increased iron in both ER+ and ER- breast cancer cells and their microenvironments has been shown to protect them from death caused by natural killer cells. Estrogen and iron appear to have synergistic effects in increasing cellular proliferation in ER+ breast cancer of BRCA1 mutation carriers.
Lavender
Lavender essential oil has estrogenic properties that sometimes can be expressed even at the modest concentrations found in personal care products.
Leatrile
Can be toxic, especially if taken orally or with vitamin C. Also known as Laetrile or vitamin B17.
Licorice root
Glycyrrhiza glabra, has both estrogenic and anti-estrogenic properties and should not be used by those with ER+ breast cancer, especially if undergoing endocrine treatment. Glycyrrhizin, the main active component of licorice, has also been shown to interfere with Adriamycin treatment.
Lindleyin
Rhubard phytochemical lindleyin is a phytoestrogen with demonstrated estrogenic activity. However, little information is available as to how it might interact with ER+ breast cancer.
Maitake mushroom
Maitake extract has been shown to have both immune enhancing and immune suppressant effects, depending on dose.
Meadow saffron
Colchicum autumnale, an unrelated and poisonous plant that should not be confused with saffron and is to be avoided. Also known as autumn crocus, colchicum or wild saffron.
Menthol
Toxic when ingested in significant amounts.
Milk thistle
Milk thistle has been shown to interfere with the effectiveness of taxane (Taxol, Taxotere) chemotherapy drugs. Treatment of mammary tumors in rodents with silymarin isolated from milk thistle stimulated cancer growth in more than one study. Milk thistle appears to have potentially harmful estrogenic effects.
Mint tea
Mint has been found to have hormonal effects at levels of consumption achievable by regularly drinking mint tea. One study found that while mint alone did not exhibit significant estrogenic activity in rats, it enhanced the estrogenic effect of estradiol when they were administered concurrently. It is possible that the combination of regular and frequent mint consumption and our estrogenic Western diet could contribute to breast cancer risk.
Momordica charantia
High dosages can cause low blood sugar and chronic use can cause kidney damage. Use only under careful supervision and monitoring. Also known as bitter melon and bitter gourd.
Ocimum basilicum
Basil (Ocimum basilicum) essential oil might cause cancer in extremely large quantities since it contains estragole. Basil pesto sauce should be limited since it can also have a significant estragole component in addition to being a significant source of copper.
Papaya seed
Papaya seed extract can be toxic, depending on dosage, due to its high benzyl isothiocyanate (BITC) content. One study found that rats consuming papaya seed extract exhibited liver cell damage and precancerous liver changes in a dose-dependent manner.
Parsley seed oil
Toxic to the liver and kidneys at high doses.
PEITC
Phenethyl isothiocyanate (PEITC) is toxic to the liver and kidneys at high doses.
Pennyroyal
Pennyroyal (Mentha pulegium), like other types of mint, has been found to have hormonal effects. Used as herbal abortifacient and to induce menses in some cultures. Can be cause liver damage, especially pennyroyal oil.
Peppermint
Mint has been found to have hormonal effects at levels of consumption achievable by regularly drinking available types of mint tea. It is possible that the combination of regular and frequent mint consumption and our estrogenic Western diet could contribute to breast cancer risk.
Phenethyl isothiocyanate
Toxic to the liver and kidneys at high doses.
Potato extract
Medicinal potato extracts containing alpha-chaconine and alpha-solanine are to be avoided since they can damage the liver.
Propolis
The chemical composition of propolis can vary greatly, depending on region and season when honey is produced, as well as supplier. Brazilian propolis has also been found to cause acute kidney failure. Propolis can sometimes induce estrogenic effects in human breast cancer cells through activation of estrogen receptors.
Protodioscin
Protodioscin has been reported to increase the levels of androgens in men.
Prunus amygdalus amara
Products incorporating Prunus amygdalus amara (bitter almond) should not be used unless prussic acid-free since they can cause cyanide poisoning.
Quercetin
Quercetin has been shown to interfere with several cytochrome P450 (CYP) enzymes and drug transporters, thereby altering the response to many chemotherapy drugs, as well as tamoxifen. Quercetin contributed to the growth of estrogen-induced mammary tumors, but only once the tumors were established in female rats, in one study.
Red clover
Red clover isoflavone biochanin A has been shown to reduce the potency of tamoxifen in rats. Formononetin, another isoflavone found in red clovers sprouts, has estrogenic properties with high affinity for ER. Both isoflavones has been shown to increase ER+/PR+ breast cancer cell proliferation in a concentration-dependent manner.
Red mold rice
Contains a mixture of monacolins, one of which is essentially identical to lovastatin, a statin used for lowering cholesterol. Therefore, red mold rice has the same potential side effects as lovastatin. The monacolin content of commercially available red mold rice supplements can vary greatly and may be lower than expected. Also known as Chinese red yeast rice.
Red sage root
Significant herb-drug interaction have been observed between red sage root (the dried root and rhizome of Salvia miltiorrhiza) and warfarin, aspirin, and digoxin, among others. CYP induction by danshen suggests that it should be avoided during tamoxifen treatment. Salvia miltiorrhiza has also been reported to have potential angiogenic properties. Also known as danshen or salvia root.
Reishi mushroom
Reishi (Ganoderma lucidum) extract has been found to have estrogenic effects, significantly increasing uterine weight in young rats in one study and promoting ER+/PR+ cell proliferation in another. Reishi has also been shown to reduce the effectiveness of Adriamycin in ER+/PR+ breast cancer cells. Reishi probably should be avoided by those with ER+ breast cancer, with the exception of patients with inflammatory breast cancer (IBC); it is one of the few foods shown to be effective in inhibiting IBC progression.
Resveratrol
Resveratrol supplementation has been shown to promote mammary tumor growth and metastasis in mouse models of ER- breast cancer.
Rhein
Rhein has been shown to interfere with several cytochrome P450 (CYP) enzymes and drug transporters, thereby altering the response to many chemotherapy drugs, as well as tamoxifen. Can cause liver toxicity at large doses or with chronic use.
Rosemary essential oil
Can cause liver and kidney toxicity if ingested at reasonably large doses or with chronic use.
Royal jelly
The chemical composition of royal jelly can vary greatly, depending on region and season when honey is produced, as well as supplier. Royal jelly has been found to have estrogenic effects in human breast cancer cells, enhancing their proliferation.
Saffron
Extract has the potential to interfere with the treatment effects of cisplatin and cyclophosphamide chemotherapy drugs.
Sage
Sage is used in some traditional medicine approaches to treat menopausal symptoms and regulate lactation. One study reported that rats' given sage extract experienced increases in the number and diameter of mammary gland ducts. Has estrogenic effects. Sage supplements should be avoided by those with breast cancer.
Sage essential oil
Can be toxic when ingested in all but very amounts.
Salvia miltiorrhiza
Significant herb-drug interaction have been observed between the dried root and rhizome of Salvia miltiorrhiza and warfarin, aspirin, and digoxin, among others. CYP induction by Salvia miltiorrhiza suggests that it should be avoided during tamoxifen treatment. Salvia miltiorrhiza has also been reported to have potential angiogenic properties. Also known as danshen, red sage root or salvia root.
Salvia officinalis
Sage is used in some traditional medicine approaches to treat menopausal symptoms and regulate lactation. One study reported that rats' given sage extract experienced increases in the number and diameter of mammary gland ducts. Has estrogenic effects. Sage supplements and sage tea should be avoided by those with breast cancer.
Salvia root
Significant herb-drug interaction have been observed between salvia root (the dried root and rhizome of Salvia miltiorrhiza) and warfarin, aspirin, and digoxin, among others. CYP induction by danshen suggests that it should be avoided during tamoxifen treatment. Salvia miltiorrhiza has also been reported to have potential angiogenic properties. Also known as danshen or red sage root.
Seaweed
Different seaweeds have varying amounts of iodine. People with thyroid disorders may find that their conditions are made worse by taking seaweed or kelp supplements, depending on the iodine content. High intake of iodine may result in elevated blood pressure and blood sugar.
Selenium
Both low and high levels of selenium are associated with heightened cancer risk. Most U.S. residents have adequate intake. Taking over 400 micrograms (mcg) per day can lead to selenium poisoning.
Soy isoflavones
Taking soy phytoestrogen supplements during tamoxifen or aromatase inhibitor treatment may promote tumor growth. Daidzein, equol or genistein supplements might also promote breast cancer growth and proliferation under some other conditions, including in women with benign breast disease and in breast cancer survivors.
Spirulina
Such supplements sometimes contain concentrated amounts of heavy metals, toxic compounds such as microcystins, and/or inedible microalgaes, depending on where the algae is grown.
St. John’s wort
St. John’s wort can interfere with the effectiveness of taxanes (Taxol, Taxotere), tamoxifen, aromatase inhibitors and many other drugs.
Sulforaphane
Isothiocyanates at doses obtainable through use of sulforaphane supplements, broccoli extracts, and other concentrated sources have the potential to damage the DNA of normal cells, thereby increasing cancer risk.
Tangeretin
Tangeretin has been shown to interfere with the effectiveness of tamoxifen.
Thyroid
Thyroid supplements taken for non-disease reasons (primarily weight loss and fertility problems) are suspected of contributing to a slight elevation in breast cancer risk. Thyroid supplements can contain unspecified iodine and/or thyroid hormone content, with unpredictable results.
Trifolium pratense
Red clover (Trifolium pratense) isoflavone biochanin A has been shown to reduce the potency of tamoxifen in rats. Formononetin, another isoflavone found in red clovers sprouts, has estrogenic properties with high affinity for ER. Both isoflavones has been shown to increase ER+/PR+ breast cancer cell proliferation in a concentration-dependent manner.
Turmeric
Turmeric and curcumin supplements should be avoided by those taking tamoxifen since they can interfere with the metabolism of tamoxifen and reduce its treatment effects (see Tamoxifen and turmeric or curcumin supplements).
Vitamin A
Use of vitamin A supplements during Adriamycin, Taxol or cyclophosphamide chemotherapy or radiotherapy has been linked to increased risk of breast cancer recurrence.
Vitamin B12
Use of vitamin B12 supplements during Adriamycin, Taxol or cyclophosphamide chemotherapy has been linked to increased risk of breast cancer recurrence.
Vitamin B17
Can be toxic, especially if taken orally or with vitamin C. Also known as amygdalin or Laetrile.
Vitamin C
Use of vitamin C supplements during treatment with Adriamycin, Taxol, cyclophosphamide, tamoxifen, or radiotherapy has been linked to increased risk of breast cancer recurrence. Use of high-dose vitamin C does not appear to be beneficial for breast cancer patients or survivors.
Vitamin E
Use of vitamin E supplements during Adriamycin, Taxol, cyclophosphamide or tamoxifen treatment or radiotherapy has been linked to increased risk of breast cancer recurrence.
White willow bark
Can be a source of cadmium exposure.
Wild ginger
Wild ginger (Asarum canadense) incorporates aristolochic acid, which has been shown to be toxic to the kidneys and is a suspected human carcinogen.
Wild Mexican yam
Wild Mexican yam (Dioscorea villosa), or wild yam, contains diosgenin, a plant steroid, which has been described as natural progesterone. While diosgenin can be converted in the laboratory into compounds such as progesterone and estrogen, it is not clear how well the body itself can make these conversions, suggesting that wild yam itself might not be very effective in alleviating menopausal symptoms. Some wild yam creams actually also contain the pharmaceutical drug progesterone, which might increase breast cancer risk. In effect, wild yam preparations with the greatest ability to offer menopausal symptom relief may be the least safe.
Wild saffron
Colchicum autumnale, an unrelated and poisonous plant that should not be confused with saffron and is to be avoided. Also known as autumn crocus, colchicum or meadow saffron.
Wild yam cream
Wild yam (Dioscorea villosa), or wild Mexican yam, is most frequently processed into a topical cream designed to alleviate symptoms associated with menopause and premenstrual syndrome. It contains diosgenin, a plant steroid, which has been described as natural progesterone. While diosgenin can be converted in the laboratory into compounds such as progesterone and estrogen, it is not clear how well the body itself can make these conversions, suggesting that wild yam itself might not be very effective in alleviating menopausal symptoms. Some wild yam creams actually also contain the pharmaceutical drug progesterone, which might increase breast cancer risk. In effect, wild yam preparations with the greatest ability to offer menopausal symptom relief may be the least safe.
Yerba maté tea
Drinking yerba maté tea has been shown to promote digestive tract cancers. This appears to be both because it normally is consumed very hot, but also because it contains relatively high levels of polycyclic aromatic hydrocarbons (PAHs), including known carcinogens. Might reduce the effectiveness of Adriamycin chemotherapy as a result of its caffeine content.
Zinc
Supplementation with zinc plus resveratrol has been shown to promote mammary tumor development.