People with heart disease are often given blood-thinning drugs such as Coumadin (warfarin) to help prevent the clots that can cause a heart attack or stroke. If you are taking one of these agents, you should not use herbal supplements, especially St. John's wort.

Researchers in the US and England studied published reports of interactions between herbal supplements and prescription drugs. They found drug interactions in nearly half of people taking warfarin and St. John's wort, primarily an increase in the anticoagulant effect of the drugs. In other words, the blood was thinned too much, which could lead to internal bleeding and difficulty clotting if you have a cut or other wound. The researchers found similar interactions between anticoagulants and danshen, dong quai, ginseng, and ginkgo.

They cautioned, "Patients on warfarin anticoagulants should be specifically advised to avoid taking herbal medicines or to have their international normalized ratio measured within two weeks of starting the product."

Related information Treatment - Medications

Source:  Dobson R. Combining anticoagulant drugs with herbal medicines is risky, report says. British Medical Journal, Dec. 1, 2001;323:1270.