A study conducted by the Medical Research Institute in New Zealand has shown that people taking the pain reliever Celebrex have nearly twice the risk of heart attacks as those using other drugs. Celebrex, which is manufactured by Pfizer, Inc., is widely used to treat the pain of arthritis. Celebrex is part of a class of drugs knows as Cox-II Inhibitors. Two other drugs in this same class, Pfizer's Bextra and Merck's Vioxx, were withdrawn from the market because of safety concerns. Vioxx was withdrawn in September 2004, after a study showed it doubled the patient's risk of heart attack and strokes after 18 months of use.