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New Study Challenges Assumptions about St. John's Wort Drug Interactions

July 12, 2000: A groundbreaking new clinical study calls into question recent warnings about potential interactions between St. John's wort and prescription drugs, including birth control pills. The new study, published in Life Sciences, showed that there were no drug interactions in people who took St. John's wort along with the anti-anxiety drug alprazolam (Xanax®) and dextromethorphan, a cough suppressant. These results refute a common but unproven assumption about St. John's wort - that the herb reduces the effectiveness of many prescription drugs because it is metabolized through the same pathway in the liver (the cytochrome P-450 system). Another significant finding of the study was the fact that St. John's wort did not interfere with 3A4, the liver enzyme used to metabolize birth control pills and other common drugs.

The new study is important because it is the first to examine St. John's wort metabolism in the human body instead of the test tube. The researchers pointed out that laboratory (in vitro) studies have "inherent limitations" that make it difficult to generalize results to human beings. "The publication of this study in Life Sciences highlights the danger of issuing public warnings about herbs based solely on in vitro evidence. It can be very misleading to extrapolate results from the test tube to the clinical realm," commented Rob McCaleb, President of the Herb Research Foundation in Boulder, CO.

No studies have yet tested the effects of St. John's wort in women taking birth control pills. One small study recently found that St. John's wort affected blood concentrations of indinavir, a drug used to treat HIV infection. Both the alprazolam study and the indinavir study had very small sample sizes of less than 10 people. In the interest of good science, researchers caution that it is difficult to make conclusions about herb safety - either negative or positive - based on small, preliminary studies. Clearly, more research is needed in the evolving science of herb-drug and food-drug interactions.

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