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St. John's wort drug interactions

Humans have used St. John's wort as an herbal remedy for thousands of years. In the past decade, though, the popularity of this "natural" medicine soared as people began using it as a treatment for mild to moderate depression and an alternative to Prozac.

As with any drug or herbal medication, users should be aware of potential negative side effects. But since St. John's wort is not a prescription drug, it never went through rigorous testing to determine these side effects before it hit over-the-counter pharmacy shelves. This information is actually just now coming to light. And the news is: Be careful what medications you take alongside St. John's wort.

After years of concern, a recent study determined St. John's wort affects the metabolism or breakdown of at least half of all drugs, reducing their effectiveness. The study specifically looked at how the remedy interacts with two enzyme subsystems, CYP 3A4 and CYP 2D6. These enzyme subsystems allow the vast majority of medications to work in the body.

In the study, twelve healthy volunteers were given two sample medications to establish a baseline for the activity of the enzymes. The volunteers then took St. John's wort (300 mg, 3 times daily) for 14 days. Follow-up assessment of enzyme activity showed the herbal remedy had no significant effect on the activity of the CYP 2D6 enzyme subsystem. However, St. John's wort was associated with a 50% reduction in the effectiveness of the CYP 3A4 enzyme subsystem. These results suggest that when taken in conjunction with St. John's wort, medications metabolized by CYP 3A4 will be half as effective and increased dosages may be required.

The consequences of taking St. John's wort with other drugs can be serious. A great number and wide variety of marketed medications are metabolized by CYP 3A4. Imagine faithfully taking your antiseizure medication for years with no problems. If you begin taking St. John's wort as well, you could suddenly experience a devastating seizure because the power of your medication is cut in half.

The bottom line is: If you use other medications, talk with your doctor and pharmacist before taking St. John's wort to find out if there is the potential for harm.

March 2004 Update

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