No social encounter delights me more than meeting a doctor at a cocktail party. In clinical settings, doctors tend to be guarded and aloof. Catch one with a whiskey in hand, though, and you might find yourself in possession of all sorts of inside information. Among the nuggets I’ve gathered in this fashion are: that salt and butter aren’t really bad for you; that nicotine is a marvelous antidepressant; that vegans are no healthier than the rest of us; and that early cancer screening may be pointless, since many small tumors vanish on their own and some grow so…