Patients taking anti-obesity drugs will only see “modest” weight loss and many will remain significantly obese or overweight, according to a study published in the British Medical Journal.

The study, which looked at the long-term effectiveness of anti-obesity medications, found that three drugs recommended for long-term use - orlistat, sibutramine and rimonabant, reduced weight by less than 11 pounds (5 kg). This equated to a loss of less than 5% of total body weight. Guidelines from the National Institute for Clinical Excellence recommend stopping the use of anti-obesity drugs if 5% of total body weight is not lost after three months.

While making changes to lifestyle and diet are recommended as the initial treatment for obesity, the use of anti-obesity drugs is common. It’s estimated that in 2005 global sales of anti-obesity drugs reached $1.2billion. Current UK guidelines recommend using drug therapy in addition to making lifestyle changes if a patient has a body mass index of greater than 30.

The Canadian researchers reviewed the evidence from thirty placebo-controlled trials where adults took anti-obesity drugs for a year or longer. The mean weight of the volunteers in all of the trials was 100kg (15.7 stone). The mean body mass index levels were 35 – 36.

Professor Raj Padwal and colleagues found orlistat reduced weight by 2.9kg, sibutramine by 4.2kg and rimonabant by 4.7kg. They also found that patients taking the weight loss pills were significantly more likely to achieve 5 – 10% weight loss, compared to those who took the placebo.

The health benefits associated with taking the drugs varied. For example, orlistat reduced the incidence of diabetes in one trial and all three drugs lowered patients’ levels of certain types of cholesterol. Adverse effects were recorded with all three drugs, in particular, rimonabant increased the risk of mood disorders such as depression or anxiety. The authors noted that no trials examined rates of death and disease as a result of taking anti-obesity pills. They recommend that trials looking at this should be carried out in the future.

The authors also noted that there were high drop-out levels in all the trials. On average 30 – 40% of patients failed to complete the trial. They say this suggests that a failure to properly adhere to the treatment could be a major factor limiting the effectiveness of anti-obesity drug therapy.

In an accompanying editorial, Professor Gareth Williams warns 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 as Glaxo Smith Kline (GSK) has applied to sell orlistat over the counter throughout Europe, it could happen here. He warns:

“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.”