TUESDAY, Jan. 8 (HealthDay News) -- An experimental weight-loss drug helped people shed pounds, but its adverse effects included gastrointestinal distress and psychiatric problems, scientists report.

Called taranabant, the drug was developed by Merck Research Laboratories, which funded the new research published in the January issue of Cell Metabolism.

"It suppresses food intake and increases your metabolism," explained Dr. Steven Heymsfield, global director of scientific affairs, obesity, for Merck Research.

The drug works by blocking the same pleasure centers in the brain that are triggered when marijuana smokers get hungry. Blocking these cannabinoid receptors reduces hunger and helps people lose weight, the researchers said.

Taranabant is in the same class of drugs as rimonabant (Acomplia), which the U.S. Food and Drug Administration has not yet approved due to concerns about the risk of suicidal thoughts among some users. Rimonabant is on the market in Europe, however.

In the new study on taranabant, "no suicidal thoughts were reported," Heymsfield said. But the question was not asked, he added, in a systemic way. For a larger, upcoming trial, it will be, he noted.

In this latest study, Heymsfield's team assigned 533 obese patients to receive either a placebo or one of four taranabant doses: 0.5, 2, 4 or 6 milligrams daily. A total of 358 patients finished the 12-week study.

While the placebo group lost about 2.6 pounds during the study, the 6-milligram taranabant group lost the most.

"Those who took the 6-milligram dose every day for 12 weeks lost about 11 pounds," Heymsfield said. They also followed a reduced-calorie eating plan but had no specific exercise plan.

In a separate study reported by the same group of scientists in the journal, the researchers gave 36 overweight or obese people a placebo or a single dose of either 4 milligrams or 12 milligrams of taranabant, or 30 milligrams of another weight-loss drug called sibutramine (Meridia). Then they tracked food intake over the next 24 hours.

Those who took the lower dose of taranabant reduced food intake 1 percent compared to the placebo group, while those who took the larger dose of the drug reduced food intake by 22 percent compared to the placebo group. The Meridia group reduced caloric intake by 12 percent.

With the higher dose of taranabant, this would drop a 2,000-calorie-a-day intake to about 1,600 calories, Heymsfield said, enough to lead to weight loss.

But as the dose of taranabant increased, so did the adverse effects, including gastrointestinal and psychiatric problems. In the 6-milligram dose group, more than 53 percent reported some sort of gastrointestinal problem, such as diarrhea, nausea, frequent bowel movements or vomiting.

And more than 27 percent of those taking that dose had psychiatric effects. Anxiety was the most commonly reported problem, but also reported were mood swings, depression, insomnia, irritability or nervousness.

Even so, Heysmfield said, the company plans to ask for FDA approval of taranabant later this year.

"It's a new class of obesity drugs, and it works through a different mechanism," he said. If approved, it would offer those who are obese and unable to lose enough weight through diet and exercise alone more options, he added. Other diet drugs work in different ways, such as decreasing the absorption of food in the gut.

One expert familiar with this class of drugs had mostly praise for the new studies. "This is a new way of thinking about regulation of food intake," said Dr. Steven R. Smith, a professor and assistant to the associate director of clinical research at Pennington Biomedical Research Center, in Baton Rouge, La.

"The paper is extremely well-written," said Smith, who was speaking on behalf of the Obesity Society. Besides reporting results, he said, the paper adds information about how the drug works.

But he said the new drug, if approved, won't be for everyone. "This is not going to be a class of drugs that people take because they want to look good on vacation" and lose 10 pounds or so, Smith said.

"I think this class of drugs, should it make it to market, is going to need to be reserved for people who have complications related to their obesity, such as type 2 diabetes, high blood pressure or severe osteoarthritis," Smith said. In general, he added, taranabant would be best for those with a body mass index of 30 or higher (classified as obese), or those with a BMI of 27, considered overweight, but with complications.

More information

To learn more about obesity, visit the Obesity Society.

SOURCES: Steven R. Smith, M.D., head, public affairs committee, the Obesity Society, and professor and assistant to the associate director for clinical research, Pennington Biomedical ResearchCenter, Baton Rouge, La.; Steven Heymsfield, M.D., global director, scientific affairs, obesity, Merck Research Laboratories, Rahway, N.J.; January 2008, Cell Metabolism

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