St. John's wort appears to interfere powerfully with a common cancer drug and can reduce its punch 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 mild depression, though its effectiveness has been questioned. Doctors also know the herb can interfere with the body's use of 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%. 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 Assn. for Cancer Research.
Despite the study's small size, experts said the findings are believable because they fit with earlier reports showing that St. John's wort can disrupt drug treatment.
Two years ago, the Food and Drug Administration warned that the herb can interfere with protease inhibitors, drugs that are widely used to treat AIDS.