Harvard Health Letter

Drug-herb interactions

Although herbal medicines are often called alternative medicine, it's not really an either-or situation. Many people take herbal medicines and mainstream pharmaceuticals.

In 2008, American Family Physician, the journal of the American Academy of Family Physicians, published an overview of interactions between drugs, herbs, and other dietary supplements. Here are four highlights of that article:

  1. Warfarin (Coumadin) seems to interact with everything: cranberry juice, fish oil, garlic, ginkgo biloba, ginseng, St. John's wort, vitamin E. But the evidence for these interactions often boils down to a few case reports. And for cranberry juice, garlic, ginkgo, and vitamin E, results from clinical trials have shown that there is little or no interaction to worry about. Still, warfarin is a tricky drug that requires close monitoring of blood coagulation, so doctors are understandably concerned when there's even a hint of something that might increase the risk of bleeding.

  2. St. John's wort may be the biggest troublemaker on the herbal side. Heart patients have to watch out: it decreases the levels of the calcium-channel blocker verapamil and of the cholesterol-lowering statins. St. John's wort is usually taken for depression, and it may work by increasing serotonin. If someone were also taking an antidepressant like fluoxetine (Prozac), the result could be excessive levels of the brain chemical, or serotonin syndrome.

  3. Ginkgo biloba, which people take in hopes of staving off Alzheimer's, may pose some bleeding risk. It has the same antiplatelet activity as aspirin and the other nonsteroidal anti-inflammatory drugs (NSAIDs).

  4. Tell your doctor if you're taking a supplement: many surveys show that over half of all patients taking both prescription drugs and dietary supplements don't. Some people may be embarrassed. Others may be reluctant to share information with a doctor who they figure will be skeptical. American Family Physician said that doctors need to take the initiative and ask patients directly whether they're taking vitamins, supplements, any special teas, or tinctures. Patients can do their part by being less reticent and telling the truth.

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