Better data on the long-term effects of anti-obesity drugs is needed before more widespread use of the therapies, a Canadian study says.
One in five adults in the UK is obese
Researchers said such drugs would become more important in the future to combat the growing obesity crisis.
But the University of Alberta Hospital said in the Lancet that data on drugs already in use was limited particularly over cardiovascular outcomes.
They said there should be better testing of anti-obesity therapies.
The team analysed articles published over the last six years on three drugs - hunger suppressant sibutramine, orlistat, which restricts the absorption of fat, and rimonabant, a relatively new drug being targeted at people with diabetes.
They said the long-term impact of the drugs was not clearly known with side effects including increased blood pressure and pulse rate for sibutramine and mood-related disorders for rimonabant reported.
The three drugs are used in the UK - where one in five adults are classed as obese - although they are restricted for clinically obese people or those at risk through conditions such as diabetes.
Doctors normally only prescribe them in tandem with exercise and dietary regimes and they are not often used for longer than a year.
But the researchers said as the obesity crisis escalates, as it is predicted to do, clinicians will have to increasingly rely on drug treatment programmes.
Lead researcher Raj Padwal said: "In light of successful weight-loss treatments and the public health implications of the obesity pandemic, the development of safe and effective drugs should be a priority."
But he said trails should show reductions in both weight loss and clinical outcomes "should be required either before these drugs are approved for widespread use or as a condition for on-going approval".
Dr Jim Kennedy, prescribing spokesman for the Royal College of GPs, said there was little known about the long-term impact of anti-obesity drugs.
But he added: "I think the use of drugs will be limited to certain situations.
"Obesity is a complex issue and there are many factors that need to be addressed, it is not just a question of handing out drugs.
"Doctors use these drugs carefully, there are no wonder cures, but in the right circumstances they can help.
"For example, if someone's diabetes is out of control or if they need to lose weight for an operation."