We tend to think of herbs as “natural” and harmless. In fact, they may interfere with cancer therapy

Leading the list in terms of popularity is garlic. Although the NCI team found that garlic affects the activity of some P450 enzymes, they concluded that no significant interactions with anti-cancer drugs are likely. However, garlic may increase bleeding when used together with aspirin, warfarin, or dipyridamole.

Then there’s ginkgo, a mainstay of Chinese medicine, used for everything from dementia and depression to vertigo and impotence. Some data suggest that ginkgo may affect the metabolism of certain cancer medications. And because of its antioxidant properties, ginkgo could, in theory, interfere with both chemotherapy and radiation therapy. (These treatments rely in part on the production of reactive oxygen—the target of antioxidants—to stop cancer cells.) Clinical studies, however, have found no evidence of this effect.

Echinacea is used mainly as a cold and flu remedy. It appears likely to interact with those anti-cancer drugs metabolized by one of the P450 enzymes.

Both soy and soy-derived products have enjoyed a growing popularity among women seeking to ease menopausal symptoms. But some animal studies indicate that soy can stimulate the growth of breast and endometrial tumors as well as neutralize the ability of the drug tamoxifen to slow breast cancer growth. For those reasons, women with estrogen-dependent tumors (as in many breast or endometrial cancers) should avoid soy.

St. John’s wort, used for medicinal purposes since the time of ancient Greece, is most commonly taken as an antidepressant. St. John’s wort appears to affect several drug-metabolizing enzymes and to interact with drugs from a variety of therapeutic classes. That suggests that taking the herb together with anti-cancer drugs should be avoided.

Saw palmetto, mainly used to reduce an enlarged prostate in men and enlarged ovaries in women, has not shown any specific drug interactions. However, the NCI team advises that people taking hormonal therapies—e.g., oral contraceptives and estrogen replacement—avoid the herb because of possible additive effects.

Ginseng has been used as a sedative, a diuretic, and an antidepressant. Because of possible effects on estrogen levels, ginseng should not be taken by women with breast or endometrial cancer.

Black cohosh, used to treat menopausal symptoms, may increase the effect of tamoxifen. While cranberry, valerian, and milk thistle have no known drug interactions, close monitoring is advised for people on anti-cancer drugs who are taking evening primrose. Individuals on chemotherapy should avoid grape seed; those who have blood clotting problems or are taking anti-clotting drugs should avoid bilberry.