For years, doctors have sought each dieter's dream of weight reduction: a pill that could peel off the pounds without any damaging effects. Their pursuit is an acknowledgment that losing weight and keeping it off isn't just a matter of willpower, instead of resisting the siren call of the ice cream in the refrigerator. New research has shown that weight reduction affects the body and brain. And even when we starve ourselves thin, we often can't undo those alterations. So we just gradually pack the pounds back because our brains think we need them.

Weight-loss pills might help. So why not more people use them?