A commonly prescribed anti-clotting drug has been found to have dangerous side effects, according to a study released early by the New England Journal of Medicine (NEJM). Clopidogrel has been hailed as one of the most effective drugs in reducing risk of heart attack and stroke caused by artherosclerosis, in which the wall of an artery thickens and loses elasticity. An estimated 3 million people have been treated with the drug worldwide. Researchers report the drug clopidogrel may be associated with a disorder called thrombotic thrombocytopenic purpura (TTP). TTP causes small blood clots to form throughout the body, leading to symptoms like jaundice, abnormal heart rhythm, and organ damage.