SAN FRANCISCO -- St. John's wort appears to interfere powerfully with a common cancer drug for weeks after people stop taking the herbal supplement, a study shows.
St. John's wort is often taken as an over-the-counter remedy for depression, though its effectiveness has been questioned. Doctors also know the herb can interfere with a variety of other medicines.
In a small study released Monday, doctors showed that St. John's wort decreases blood levels of one chemotherapy drug by about 40 percent. This effect lingered for more than three weeks after people stopped taking the supplement.
Dr. Ron Mathijssen of the Rotterdam Cancer Institute in the Netherlands, who directed the study, said St. John's wort could reduce the ability of chemotherapy to knock down cancer.
''People don't realize it is a drug because you don't need a prescription. People think it's harmless,'' he said.
Mathijssen presented the results of the study, conducted on five patients, at a meeting in San Francisco of the American Association for Cancer Research.
Despite the small size of the study, experts said the findings are believable because they fit with earlier reports.
Use of supplements is common in cancer patients. Some studies suggest that about half of them use vitamins and other over-the-counter treatments in hopes of improving their chances of beating the disease.
St. John's wort interferes with an enzyme called P450 that the body uses to break down about half of all drugs. Because of this, St. John's wort is believed to inhibit many of the most widely prescribed medicines. Among others are digoxin and beta blockers used for heart disease, seizure medicines and drugs used to prevent organ rejection after transplants.
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