St. John’s wort weakens cancer drug

Written by pharmacist  |  10. April 2002

Many of us in the health-care industry know that the over-the-counter 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 a chemotherapy drug used to treat many different types of cancer. 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. Three patients were exposed to a normal course of the cancer drug Camptosar, known generically as irinotecan, followed three weeks later by a second course that combined the drug and St. John's wort. The two others received the combination treatment, and three weeks later received irinotecan alone to see whether the herbal supplement still had an effect. The study found that the body's exposure to the cancer-killing chemotherapy fell by about 40 percent when St. John's wort was taken. The researchers also found that the effect lasted for more than three weeks after the patients stopped taking the herb. "This means people have to realize that it's not good enough to stop using St. John's wort just prior to treatment with irinotecan," Mathijssen said. The study was not designed, however, to determine how long before starting chemotherapy patients need to stop taking herbal supplements. Despite the small size of the study, 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. Among others, experts say, are digoxin and beta blockers used for heart disease, seizure medicines and drugs used to prevent organ rejection after transplants. HERBS POPULAR AMONG CANCER PATIENTS About half of all cancer patients take some kind of health supplements, including sometimes "industrial strength" vitamins, said Dr. Karen Antman, director of medical oncology at New York's Columbia-Presbyterian Hospital, "We are very concerned in the medical community about the drugs our patients are taking that we don't know about," Antman said. "I make it a point to ask, but I'm not sure my patients always tell me the truth." St. John's wort stimulates an enzyme called P450 that is involved in the metabolism, through the liver, of about half of all drugs, including common chemotherapy agents. "Many people who have cancer become mild to moderately depressed and I can understand that. But it might be safer to have them on a drug we know more about," Antman said.

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