Pfizer ( PFE ) shares fell Thursday following a Food and Drug Administration warning about company advertising for two controversial pain drugs. The FDA sent a letter to the New York drugmaker stating that television and print ads for the drugs Bextra and Celebrex were misleading. The letter said the ads did not fully disclose side effects while also making "unsubstantiated effectiveness claims." Bextra and Celebrex -- like Merck's ( MRK ) Vioxx -- are pain killers that belong to a family of drugs known as COX-2 inhibitors and have been the subject of safety concerns. Merck withdrew Vioxx in late September because a company study showed its use increased the risk of heart attacks in some patients. Pfizer has stood by the safety of Celebrex but has said it will do additional testing. The FDA recently decided Bextra needed tougher labeling because of links to a rare, aggressive and sometimes fatal skin rash. The FDA letter said five ads "omit material facts" and "make misleading safety, unsubstantiated superiority and unsubstantiated effectiveness claims." Pfizer shares fell 60 cents, or 2.3%, to $25.43. Safety concerns about COX-2 drugs have dragged down shares of Pfizer and Merck in the past four months.