For the first time, leading prostate cancer specialists say, they have a drug that can significantly cut men’s risk of developing the disease, dropping the incidence by 30 percent. But the discovery, arising from a new analysis of a large federal study, comes with a debate: Should men take the drug? Prostate cancer is unlike any other because it is relatively slow-growing and while it can kill, it often is not lethal. In fact, most leading specialists say, a major problem is that men are getting screened, discovering they have cancers that may or may not be dangerous, and opting...