St. John's wort hinders cancer drug. (Chemotherapy).

St. John's wort, the popular herbal remedy for mild depression, lowers the effectiveness of a cancer-fighting drug, according to a study in the Netherlands.

This previously unrecognized downside results from the herb's stimulation of an enzyme in the body called cytochrome P450. That enzyme breaks down certain drugs, including irinotecan, a chemotherapy drug used against various cancers, says Ron A.H.J. Mathijssen, a physician at the Rotterdam Cancer Institute.

St. John's wort contains two compounds--hypericin and hyperforin--that induce cytochrome P450 to increase the rate of drug metabolism. To test the herb's effect on irinotecan, Mathijssen and his colleagues started giving daily doses of St. John's wort to three cancer patients just before a chemotherapy infusion. Subsequent blood tests showed that the anticancer drug's activity declined by 40 percent. Even when the patients took the herb 3 weeks before their chemotherapy, it had drug-inhibiting effects, Mathijssen says.

"We know that about 50 percent of cancer patients use alternative medicines, and many don't tell their doctors," says Karen Antman, a physician at Columbia University. If larger tests show that St. John's wort and other over-the-counter supplements interfere with chemotherapy, the Food and Drug Administration might need to require warning labels on them, Antman says. --N. S.