The rate of weight loss with Lorcaserin is modest compared to other drugs in development, says Sajani Shah, an obesity expert at Tufts Medical Center, in Boston, who was not involved in the study. "But this drug looks more safe so far," she says.

According to the study, the most common side effects were headaches, dizziness, and nausea. Most significantly, scientists did not see an increase in heart-valve disease in people who took Lorcaserin, though the researchers will need to show this is true in a larger number of patients in order to satisfy the U.S. Food and Drug Administration. The drug also appears to lack the psychiatric side effects, such as depression, seen with some other weight-loss drugs. Japanese drugmaker Eisai bought marketing rights to the drug on July 1.

Weight-loss drugs have had a troubled past. The field's greatest recent hope, a drug called rimonabant, was pulled from the European market in 2007 due to an increased risk of depression and suicidal thinking. (It was not approved in the United States.) A second drug, called sibutramine, was withdrawn in Europe this year after being linked to an increased risk of stroke and heart attack. (The FDA plans to review the drug later this year.) And in May, the FDA warned that a drug called orlistat, sold over the counter as Alli, is linked to a rare type of liver injury.

Lorcaserin is one of three experimental weight-loss drugs the FDA will review this year, and the only novel compound in the group. The other two drugs, Qnexa and Contrave, are both combinations of existing drugs originally designed to target addiction, depression, and obesity. An FDA committee reviewed data on Qnexa, a combination of the weight-loss drug phentermine and the anticonvulsant topiramate, this week. A report released Tuesday concluded that the drug is effective but has some safety concerns. For example, clinical studies show it is linked to mild to moderate psychiatric side effects, such as depression.