Several prescription weight loss drugs have been available in the United States. The Food and Drug Administration approved phentermine in 1959 and approved fenfluramine in 1973 for short-term use (a few weeks). In 1996, dexfenfluramine was approved for longer use (up to 1 year). In 1997, sibutramine was approved for short-term use. These four drugs work by limiting hunger. Orlistat, a drug that blocks the amount of fat absorbed from food, was approved in 1999 for up to 2 years of treatment. Fenfluramine or dexfenfluramine was often used in combination with phentermine; this combined weight loss treatment is known as “fen-phen.” Reports of heart valve problems in “fen-phen” users led the manufacturer to stop selling fenfluramine and dexfenfluramine in the United States in September 1997. Little is known about how many people actually take these different drugs or how overweight these persons are when they start taking them.