The users call the drug Lizzie, the Big Brother or sometimes Gilly. On blogs they rave over its uncanny ability to melt away pounds, although some are wary of its side effects, which can include nausea and strange welts. The users are not fad dieters or methamphetamine addicts, but people with diabetes. And the subject of their rhapsodies is not a gray-market diet pill sold on late-night television but Byetta, a federally approved diabetes medicine, available only by prescription, whose popularity and sales have soared since its introduction last June. For diabetics, the weight loss caused by Byetta comes as...