WASHINGTON (AP) - The painkiller Vioxx, pulled from the market last fall because of reported heart problems and strokes in users, could come back on sale if federal regulators conclude it and similar drugs have benefits that balance the risks. Dr. Peter S. Kim, president of Merck Research Laboratories, told a joint meeting of two Food and Drug Administration advisory committees on Thursday that new studies indicate the side effects aren't unique to its product. The advisory panels were concluding a three-day meeting Friday in which they have been assessing drugs known as Cox-2 inhibitors, which also include Pfizer Inc....