Though most users of the drugs remained overweight, experts said the drugs could help curb the dangers of obesity by reducing rates of heart disease, diabetes and other health problems.
In a paper published Friday in the British Medical Journal, researchers in Canada and Brazil analyzed existing data on three popular weight-loss drugs: orlistat, or Xenical; sibutramine, known as Meridia in the United States and Reductil in Europe; and rimonabant, or Accomplia.
Scientists found that patients on the drugs men and women between 45 and 50 years old who weighed about 100 kilograms (220 pounds) and had a body mass index of about 35 lost less than 5 kilograms (11 pounds) on average. The study participants used the drugs for periods of between one and four years.
"Drugs are not the magic cure and are not for everybody," said Dr. Raj Padwal, an assistant professor at the University of Alberta in Canada, one of the paper's authors. "But in specific patients, they have great benefits."
Padwal and colleagues considered 16 trials that tested orlistat, which involved 10,631 people. Orlistat, which works by preventing fat digestion, helped people lose about 3 kilograms (6.6 pounds) on average. But it also reduced diabetes and improved their cholesterol levels and blood pressure. Up to 30 percent of patients had unpleasant digestive and intestinal side effects, such as incontinence.
Of the 10 trials on sibutramine, which involved 2,623 people, study participants lost about 4 kilograms (9 pounds) on average and had improved cholesterol levels. In up to 20 percent of patients, sibutramine caused side effects including raised blood pressure and pulse rates, insomnia and nausea.
And in the four rimonabant studies involving 6,365 people, scientists found that users lost on average about 5 kilograms (11 pounds). Rimonabant also improved their blood pressure and cholesterol levels. The risk of mood disorders increased in 6 percent of patients.
Both sibutramine and rimonabant work by interrupting nerve signals in the brain.
Another study published Friday in The Lancet also showed rimonabant raised the risk of psychiatric problems like depression and anxiety.
A report from the United States' Food and Drug Administration in June found that 26 percent of people on rimonabant versus 14 percent of those given a placebo developed symptoms that included depression, anxiety and, in severe cases, suicidal tendencies. The FDA refused to authorize the drug.
Rimonabant has been approved by the European Drug Agency, and is available in countries including Austria, Denmark, Germany, Greece and the United Kingdom.
Sibutramine and orlistat are licensed for sale in the U.S. and Europe. Another version of orlistat known as Alli is sold over the counter in the U.S., and its maker, GlaxoSmithKline PLC, is seeking approval for sales in Europe.
Some experts say that the few pounds the drugs help people to shed are worth it. "Modest weight loss brings surprisingly big health gains," said Susan Jebb, head of nutrition and health at Britain's Medical Research Council. Jebb was not tied to either study.
"We are not just fighting obesity, but the things that come along with it," Jebb said. Losing as little as 5 pounds (2 kilograms) can help reduce the risk of heart disease and diabetes.
But other experts worry that easy access to diet drugs give people a false sense of security.
"Selling anti-obesity drugs over the counter will perpetuate the myth that obesity can be fixed simply by popping a pill," wrote Dr. Gareth Williams, dean and professor of medicine at the University of Bristol, in an editorial in the British Medical Journal.
Padwal said the biggest caveat about the drugs is that their long-term effects are unknown. In 2005, global sales of the drugs were estimated at US$1.2 billion.
Faced with an increasing global obesity epidemic the World Health Organization estimates that 3 billion adults will be overweight or obese by 2015 many experts think the drugs could be used more widely.
"Diet and lifestyle interventions on their own have been stunningly poor," Jebb said. "We've got to be realistic," she said. "Even though the weight losses from the drugs are modest, they're better than most other things we've got."