Anti-obesity medications can only help obese patients lose a "modest" amount of weight, report Canadian researchers in a review of a group of studies on the long-term effectiveness of the drugs.

The researchers from the University of Alberta and the University of Calgary reviewed 30 placebo-controlled studies in which adults took anti-obesity drugs for a year or longer.

They looked at three drugs recommended for long-term use:

• Orlistat (sold as Xenical in Canada and as Alli in the U.S.), which blocks the digestion of dietary fat
• Sibutramine (Meridia) an appetite suppressant
• And rimonabant (Acomplia), which limits the amount of fat the body can absorb (It is not approved for sale in Canada or the United States)

The mean weight of the volunteers in all of the trials was 100 kg (220 lbs). The mean body mass indexes (BMI) were 35 to 36. A BMI between 19 and 25 is considered normal.

The review, published in the British Medical Journal, found that, on average, the drugs reduced weight by less than 5 kilograms (11 pounds). This equated to a loss of less than five per cent of total body weight for the users who took part in the study.

They found:

• Orlistat reduced weight by 2.9 kg (6.4 pounds)
• Sibutramine by 4.2 kg (9 pounds)
• Rimonabant by 4.7 kg (about 10.5 pounds)

The researchers did find that patients taking the drugs lost more weight compared to those taking placebo pills. But the total lost amounted to only about five to 10 per cent of the starting body weight. And the authors note that current guidelines from the National Institute for Clinical Excellence in the U.K. recommend stopping anti-obesity drugs if five per cent of total body weight is not lost after three months.

The drugs provided some additional health benefits, the authors say, but those benefits varied. For example, all three drugs lowered patients' levels of certain types of cholesterol. As well, orlistat reduced the incidence of diabetes in one study they examined.

But there were also side effects noted with all three drugs -- in particular, rimonabant. The researchers found that drug increased the risk of mood disorders such as depression or anxiety.

Coincidentally, a study was published Friday in The Lancet that also looked at the ill effects of rimonabant. It showed that obese patients taking the drug have a 40 per cent increased risk of developing severe depression and anxiety compared to those taking a placebo.

The researchers in the BMJ review also noted that there were high drop-out levels in all the studies they examined. On average 30 to 40 per cent of patients failed to complete the studies they were involved in.

They say a failure to properly adhere to the treatment could be a major factor limiting the effectiveness of any anti-obesity drug.