Fosamax is one of the most-prescribed drugs in America, as millions have taken or are now taking it to prevent bone loss in men and women, due primarily to osteoporosis. Fosamax now faces mounting concerns that its use may lead to a serious side effect called osteonecrosis of the jaw, also known as Dead Jaw or ONJ. Dead Jaw happens when the jaw bone tissue dies. People who have dental work done while taking Fosamax are mostly likely to be at risk.