A study led by a UT Southwestern Medical Center researcher indicated on the brain chemical serotonin, when spurred by diet drugs such as Fen-phen, works to curb appetite. This knowledge could aid in the design of safer anti-obesity drugs nearly a decade after Fen-phen was banned for causing harmful side effects. The study found that serotonin activates some neurons and melanocortin-4 receptors, or MC4Rs, to curb appetite and at the same time blocks other neurons that normally act to increase appetite.