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Research Brief

The prevalence of overweight and obesity[1] is rising precipitously worldwide: A 1999–2002 survey estimated that 30 percent of the U.S. population is obese, the result of as-yet-unexplained interactions between heredity and the environment. The health consequences of obesity are considerable, yet weight loss of just 5 to 10 percent may lower the risks. Some 40 percent of Americans are trying to lose weight, accounting for the increased number and popularity of prescription weight-loss medications. To assess the effectiveness and safety of these drugs, researchers at the Southern California Evidence-Based Practice Center, which is funded by the Agency for Healthcare Research and Quality and housed at RAND, analyzed 78 published studies of the most popular diet drugs.

  • Among the prescription medications approved by the U.S. Food and Drug Administration for weight loss, all are appetite suppressants except orlistat, which inhibits the digestion and intestinal absorption of some dietary fats, including fat-soluble vitamins.
  • Twelve months of treatment with sibutramine or orlistat, the two most well-studied prescription weight-loss medications, promoted moderate weight loss (11 pounds or less over a year) compared with a placebo pill, but only when prescription of the drugs was accompanied by diet recommendations.
  • Other medications prescribed for weight loss—including phentermine, buproprion, and topiramate, and probably diethylproprion and the antidepressant fluoxetine—also promoted moderate weight loss when prescribed along with diet recommendations.
  • Few of the studies assessed whether the drugs improved medical problems associated with obesity, but people who lost weight using the drugs were less likely to develop diabetes; and weight loss has been associated with improvements in high blood pressure, blood sugar, and cholesterol levels in other studies.
  • Side effects were reported for each of the drugs, including insomnia and gastrointestinal upset, but not enough information was available to determine whether the medications might pose any long-term health problems.
  • Too few studies included children or adolescents to allow any conclusions to be drawn about the effectiveness or safety of prescription weight-loss medications for these populations.


  • [1] Individuals whose body mass index (BMI: a ratio of weight to height) falls between 25 and 29.9 are considered overweight. Individuals whose body mass index is 30 or higher are considered obese. For example, a person with a BMI of 30 who is 5 feet 10 inches weighs 209 pounds.

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