Diet drugs like Acomplia (rimonabant), Xenical and Meridia that patients may need to take for years require more study to see if they also help people live longer before doctors can be sure the benefits are greater than the risks, according to Canadian researchers writing in The Lancet.

While Acomplia, which is not yet approved in the United States, appears to help patients lose slightly more weight than Xenical or Meridia in the short term, clinical trial results suggest that patients will quickly regain the weight they lost unless they keep taking it indefinitely.

Researchers Raj Padwal and Sumit Majumdar of the University of Alberta Hospital in Edmonton said "the neurobiology of obesity is extremely complex," and not enough is known about clinical causes of obesity -- or long term effects of diet drugs -- to conclude that it is safe to take these pills for years.

They noted that research on diet drugs has been plagued by high drop-out rates, and by a lack of data on long-term illnesses and deaths.

"The lack of cardiovascular morbidity and mortality endpoints in obesity drug trials represents a major gap in knowledge,'' the researchers wrote.

"There are no definitive data showing benefit of one anti-obesity drug over another, and all three drugs are limited by modest efficacy and low rates of persistence with treatment,'' the Canadian researchers said.

In the absence of such data, they concluded that obesity-drug trials "should be required either before these drugs are approved for widespread use or as a condition of ongoing approval" to show that the drugs provide vital benefits beyond modest weight loss.