Drugs that target the brain to promote weight loss have two potential weaknesses.

For starters, because they act on a complex organ with many other functions, these drugs can trigger a wide array of unforeseen side effects. Second, scientists increasingly suspect that the brain is not the controller of appetite-related signals, but a switching station for them; if that’s the case, then drugs that work on the brain are more easily circumvented by the body’s natural tendency to hold onto or regain fat.