Anti-obesity drugs work in one or more of the following ways. They can suppress the appetite, increase the body's metabolism or interfere with the body's ability to absorb specific nutrients in food.
GSK has sold about $300 million of its diet pill Alli (Xenical) in the U.S. since June 2007, when the drug went on sale without a prescription. 
Sanofi, France's biggest drugmaker, withdrew its application to sell rimonabant in June 2007 after an FDA recommended it not be approved following the suicides of three patients. The drug is sold outside the U.S. as Acomplia.
Pfizer Inc. is also developing a drug that targets the same receptor in the brain as Merck and Sanofi's. New York-based Pfizer's pill is about two years behind Merck's in testing.