What is the difference in fenfluramine and Meridia?

Fenfluramine is a neurotransmitter releaser while Meridia only inhibits reuptake

Studies have shown that Meridia® is moderately effective in producing weight loss in obese individuals, with 85 percent losing at least 5 percent of body weight and 57 percent at least 10 percent. In the majority of studies it has not been shown to be as effective as the combination of fenfluramine and phentermine ("phen/fen") which enhances the release of norepinephrine and serotonin. Subjects in studies by Weintraub and Wadden lost an average of 15.9 percent of body weight. However, with the withdrawal of fenfluramine from the market in 1997, the combination is no longer available.

Meridia®  only works on clearance of serotonin. Fenfluramine and dexfenfluramine also enhance its release. A theory has been proposed that serotonin releasing drugs, like fenfluramine and dexfenfluramine, which activate the 5-HT2B receptor, may contribute to the development of heart valve disease.