Many people use both herbal therapies and prescription drugs, but little information is available about interactions between these 2 types of therapy. Warfarin is a commonly used anticoagulant drug. Doctors prescribe these drugs to prevent unwanted blood clots from forming in blood vessels or in the heart or to prevent existing unwanted blood clots from getting larger. Some people call anticoagulant drugs “blood thinners.” Many things can influence the effectiveness of warfarin, including certain drugs, foods, and herbal therapies. Changes in warfarin's effectiveness have been reported in patients who were taking herbal therapies while receiving warfarin. Some people are concerned that ginseng, a popular herb, might interact with warfarin to decrease its ability to prevent clots, but no study has shown a ginseng–warfarin interaction.