Pills are no substitute for a healthy diet and lifestyle
Patients taking anti-obesity drugs lose only "modest" amounts of weight, and many remain significantly obese or overweight, research reveals.
Fat pills like orlistat reduced weight by less than 5kg (11 pounds) or 5% of total body weight - which guidelines say makes their use unjustified.
Experts said the Canadian work in the British Medical Journal shows pills are no substitute for healthy living.
Eating less and exercising more is essential, they said.
Over a billion people worldwide are overweight or obese, making the anti-obesity drug market big business.
An estimated $1.2 billion was spent on anti-obesity drugs worldwide in 2005.
The latest work by Professor Raj Padwal and his team at the University of Alberta suggests in many cases these pills achieve little in terms of weight loss.
They reviewed the evidence from thirty placebo-controlled trials, involving nearly 20,000 people, where adults took one of three anti-obesity drugs - orlistat, sibutramine or rimonabant - for a year or longer.
The National Institute for health and Clinical Excellence recommends stopping the use of anti-obesity drugs if 5% of total body weight is not lost after three months.
The majority of us would be obese by 2050, forecasters predict
All of the volunteers in the trials were deemed obese, and weighed an average of 100kg (15.7 stone).
Orlistat reduced weight by 2.9kg, sibutramine by 4.2kg and rimonabant by 4.7kg.
Patients taking the weight loss pills were significantly more likely to achieve 5%-10% weight loss, compared to those who took a dummy drug, however.
But is was unclear whether the weight loss achieved was enough to have big health and survival benefits.
Orlistat reduced the incidence of diabetes in one trial and all three drugs lowered patients' levels of certain types of cholesterol.
But adverse effects were recorded with all three drugs and there were high drop-out rates, with 30%-40% of patients failing to complete the trials.
A separate study in The Lancet found patients given one of these weight loss drugs, rimonabant, were at increased risk of severe psychiatric events.
Dr Colin Waine, chair of the National Obesity Forum, said: "The first choice has to be a healthier lifestyle, but medication use can be justified because obesity is a serious medical condition.
"The purpose is not just to achieve weight loss but to reduce health risk factors. All three drugs can have beneficial effects on those."
Professor Gareth Williams, professor of medicine at the University of Bristol, warned of the potential damage to society if anti-obesity drugs were licensed to be sold without prescription, as already happens in the US.
"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.
"Globally, obesity is spiralling out of control and will only be reined in by public health campaigns that somehow persuade people to eat less and exercise more."