America is in the midst of a major obesity epidemic, with two-thirds of the population either overweight or obese.

Drugs that could safely help people lose weight would have a significant impact on public health. But do they exist?

Effective weight loss drugs used to be available (amphetamines, phentermine and fenfluramine) but were pulled from the market years ago by the Food and Drug Administration because of severe side effects such as heart valve damage, psychosis and addiction.

Current weight-loss drugs include orlistat (Xenical), sibutramine (Meridia) and rimonabant (Acomplia).

The first two are approved in the United States, while the third is approved in Europe. In a review of literature, Canadian researchers assessed the effectiveness and side effects of these drugs. Each produces only modest weight loss and none is superior to the others.

All have side effects that can be uncomfortable or dangerous; orlistat can cause gastrointestinal distress and diarrhea, sibutramine increases blood pressure and rimonabant can trigger insomnia, depression and psychological disturbances.

Not much data exists on the long-term safety of these weight loss drugs. The authors concluded that not enough is known about the mechanisms of obesity to prescribe drugs to overweight people for extended periods. [sources: Lancet, Fitness RX]