Merck Research Laboratories developed a new drug, called taranabant, which helped people shed pounds, but taken in higher quantities can lead to gastrointestinal distress and psychiatric problems, scientists said.

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

The pill is currently being tested in a far larger late-stage trial whose results are expected to be unveiled later this year.

Taranabant, as well as Sanofi-Aventis’s rejected ribonabant, works by blocking cannabinoid receptors in the brain, which receptors make people hungry when smoking marijuana.

"When you block the cannabinoid system with an antagonist like taranabant, you suppress appetite," said Heymsfield.

The study included 553 obese patients, who were divided in more groups, which were administered 0.5,2,4 or 6 milligrams daily. The last group was given placebo. The patients were followed for a period of 12 weeks. They also followed a reduced-calorie eating plan but no specific exercise plan.

"Those who took the 6-milligram dose every day for 12 weeks lost about 11 pounds," Heymsfield said.

Those on placebo lost only 2.6 pounds he added.

The researchers discovered that people who took the drug ate 20 percent fewer calories than those who were given placebo. The drug also increased resting energy expenditure or metabolism by 5 percent. The bad part of the study is that 30 percent of people who received taranabant reported psychiatric-related adverse events including depression and anxiety, compared with 18 percent in the placebo group. Larger amounts also led to stomach upsets, nausea and vomiting and caused patients to become more irritable.

Taranabant comes second after Sanofi-Aventis’ rimonabant(Acomplia) which was rejected by the Food and Drug Administration in the United States due to concerns about the risk of suicidal thoughts among some users. However, rimonabant is on the European market, being available in some countries. Its manufacturer is facing two class-action lawsuits filed by investors because Sanofi allegedly released misleading information about the depressive effects of the drug.

"In doses we're studying, we believe the drug can be effective in weight loss, yet be safe," said Merck spokesperson Amy Rose.

However, she added that the safety of the drug would be checked in a larger Phase III trial, as "we need to wait for reliable data." Rose said that much data from the actual study was presented in October at a medical meeting in New Orleans.

According to the Centers for Disease Control and Prevention, two-thirds of the U.S. population is overweight or obese.

Merck’s representative Ian McConnell said last month that it expects to file for marketing approval of the drug by the end of 2008.

"Two hundred million people could be candidates to take this drug. I think the FDA is being very, very careful," said Raj Padwal, an assistant professor at the University of Alberta.

The study was published on Tuesday issue of the Cell Metabolism.

Merck, based in Whitehouse Station, New Jersey, rose $1.74, or 3 percent, to $59.66 at 4:05 p.m. in New York Stock Exchange composite trading, the biggest increase since October. The stock has increased 35 percent in the past 12 months.