Just because something is natural doesn't mean something is safe. To measure the effect of St. John's wort on the metabolism of drugs, Gurley analyzed how long the body holds onto the drug midazolam. He gave 12 college students 300 milligrams of St. John's wort three times a day for 28 days and then gave them an oral dose of midazolam. An hour later, tests showed that the students' bodies had metabolized the midazolam more than twice as quickly as they had in tests before the St. John's wort regimen began, he says. By contrast, when the students were put on similar regimens of garlic and ginseng, the study says, metabolism of the medicine was not affected.
Researchers remain unsure just what components of herbs are responsible for their different effects.
Most herbs are a composite of various plant chemicals, and many of those have strong medicinal effects, Gurley says. Plus some herb mixtures contain elements such as lead and mercury, he says, so you are concocting combinations of medicines with truly unknown consequences. And doctors need to educate themselves about supplements so they can ask the right questions, Druss says. To read more about St. John's wort, visit the National Institutes of Health and the National Institute of Mental Health online. Or, you may want to read previous HealthScout articles on St. John's wort and other herbal supplements .