Effective obesity drugs have proven enormously difficult to develop. The brain circuits responsible for appetite overlap with those that control other important functions, including mood, raising the risk of side effects. And obese patients would probably have to take a drug for years, with testing involving large patient populations, driving up development costs. Stephen Bloom, an endocrinologist at Imperial College London, says: "The obesity problem is unsolved, and looks like it's going to stay that way for quite some time."