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.
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.