Patients taking diet drug rimonabant can expect to see a only a "modest" weight loss of less than 5 percent of their total body weight and most will remain obese or significantly overweight, according to a study published on the British Medical Journal website.

The study by Canadian researchers, which also looked at the long-term effectiveness of prescription drugs orlistat (Xenical) and sibutramine (Meridia), found that none of them reduced the body weight of the typical trial participant by 5 percent.

Guidelines from the U.K.'s National Institute for Clinical Excellence (NICE), which makes recommendations to the Britain's National Health Service, recommend halting use of diet drugs if 5 percent of total body weight is not lost within three months.

The Canadian researchers reviewed the evidence from thirty placebo-controlled trials where adults took anti-obesity drugs for a year or longer -- including the four major clinical trials that resulted in approval of sale of rimonabant in the European Union but not in the United States.

Professor Raj Padwal and colleagues found that rimonabant (Acomplia / Zimulti) resulted in a somewhat larger weight loss than Xenical or Meridia, but noted that adverse effects were recorded with all three drugs and in particular, with rimonabant.

"The most worrying adverse effect was an increased incidence of psychiatric disorders (depression, anxiety, irritability, aggression), which occurred in 6 percent of patients receiving rimonabant and was 3 percent more likely in patients receiving rimonabant compared with placebo," the researchers reported.

"The increased incidence of mood disorders with rimonabant indicates careful surveillance, particularly because psychiatric illness commonly coexists with obesity," the researchers said.

"As the patients enrolled in the rimonabant trials were carefully screened to exclude those with major psychiatric disease, the risk of mood disorders with rimonabant might be underestimated," they added.

Sanofi-aventis, in a statement issued on Nov. 16th commenting on the report in the British Medical Journal and another which appeared in the Lancet, said "the clinical data resulting from these meta-analyses do not add any new information and are in line with the clinical trials on efficacy and safety known with rimonabant today.

"However, the extrapolation of certain data appears to us to be the sole result of the authors’ opinion," the company said.

In an editorial that accompanied the British Medical Journal report, Professor Gareth Williams also warned of the potential damage to society if anti-obesity drugs are licensed to be sold without prescription.

This already happens in the United States, and where an low-dose version of orlistat (alli) has been approved by the FDA for over the counter sale, and the manufacturer is also hoping to sell alli over the counter throughout Europe.

"Selling anti-obesity drugs over the counter will perpetuate the myth that obesity can be fixed simply by popping a pill and could further undermine the efforts to promote healthy living, which is the only long term escape from obesity," Williams concluded.