Herbal Drugs: Reactions with other drugs or substances

Last modified: Sunday, 31. May 2009 - 4:03 pm

One in six patients taking prescription drugs is also taking one or more herbal drugs, according the Slone survey published in JAMA. The study’s authors concluded that this situation creates a high potential for adverse drug-herb interactions. Although most herbals have not been studied for possible interactions with prescription drugs, a few researchers are beginning to explore this potential problem. Following is a list of some herbal drugs commonly used in the United States and what is known about their interaction with conventional drugs.
Cranberry (Vaccinium macro carp on). Excessive intake of cranberry juice, used to treat urinary tract infections, may reduce the blood levels of certain drugs, including some antidepressants, antipsychotics, and morphine-based painkillers.
Dong quai (Angelica sinensis). One study in rabbits suggests that this herb can enhance the blood-thinning effect of the drug warfarin, although dong quai does not appear to have any blood-thinning ability on its own. (Herbalists believe dong quai relieves menstrual cramps, pre-menstrual syndrome, and many symptoms of menopause.)
Echinacea. Studies have so far found no drug interactions for this herb. In Germany, echinacea is often combined with antibiotics to treat bacterial infections, although no studies have evaluated the safety or effectiveness of this combination.
Garlic. According to a study from the National Institutes of Health, garlic supplements sharply decrease the amount of the anti-HIV drug saquinavir in the body, presumably reducing the drug’s effectiveness. In addition, because of garlic’s apparent blood-thinning properties, it should be used with caution by patients who are already on blood-thinning drugs such as warfarin. One study indicated that patients taking both warfarin and garlic took longer to stop bleeding.
Ginkgo. Because this herb can inhibit blood platelet activity, it should be used cautiously with blood-thinning drugs such as warfarin, heparin, or even aspirin.
Ginseng. This herb may react with the drug phenelzine, causing headaches, tremors, and manic-like symptoms, according to reports. However, it is possible that the ginseng in these cases may have been contaminated with caffeine. Ginseng also lowers blood concentrations of alcohol and warfarin.
Kava. This anti-anxiety herb may react synergistically with (enhancing the effect of) drugs that affect the central nervous system, such as alcohol, barbiturates, or prescribed anti-anxiety drugs. One patient was hospitalized from a reaction between Xanax and kava extract. Herbalists often recommend combinations of kava and St. John’s wort to treat anxiety, but the safety of this combination has not been established.
St. John’s wort. Preliminary reports indicate that St. John’s wort may interfere with the anti-tumor drugs etoposide (VePesid), teniposide (Vumon), mitoxantrone (Novantrone), and doxorubicin (Adriamycin). This herb also lowers the amount of indinavir, an anti-retroviral drug used to treat HIV/AIDS, in the blood. Because St. John’s wort likely reduces the effect of other anti-retroviral drugs as well, people being treated for AIDS should avoid this herb.
Valerian {Valerian offtcinalis). No drug interactions have been reported, but animal studies suggest that valerian, used as a sedative and sleep aid, might increase the effects of barbiturates such as pentobarbital, hexobarbital, and thiopental.

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