People who are seriously overweight should only be given anti-obesity drugs if they are prepared to diet and exercise, say doctors.
One in four Britons is clinically obese
A report by the Royal College of Physicians (RCP) also suggests that patients should be taken off the drugs if they fail to lose enough weight.
Obesity levels across Britain have jumped sharply over the past 20 years. An estimated 55% of people are now either overweight or clinically obese.
More and more patients are being prescribed anti-obesity drugs in an attempt to tackle the problem.
'No quick fix'
However, guidelines from the RCP warn doctors that these drugs should not be seen as a "quick fix".
The report states: "It is important to avoid offering anti-obesity drug therapy to patients who are seeking a 'quick fix' for the weight problem."
The guidelines suggest that doctors should require patients to diet and exercise for three months before they are offered the drugs.
"The accepted first line strategy for weight reduction and weight maintenance is a combination of diet, exercise and behaviour modification, lasting at least three months," they state.
The report adds that doctors should only prescribe these drugs to patients sooner in exceptional circumstances.
It also states that patients should stop taking the drugs if they fail to achieve a five percent weight reduction after 12 weeks.
"Not all obese patients respond to drug therapy," the report says.
Professor Peter Kopelman, chairman of the RCP's nutrition committee which drew up the report, said the guidelines would help doctors decide who should receive the drugs.
"The report provides up-to-date guidance for all medical practitioners about the appropriate use of anti-obesity drugs for the management of overweight and obese patients," he said.
"It places the prescription of such drugs into the context of good medical practice and makes strong recommendations about the initiation of treatment and follow-up arrangements for patients."
More Britons are now clinically obese than ever before. In 1980, 6% of men and 8% of women were obese. In 2000, that figure had jumped to 21% for both sexes.
Doctors blame changes in lifestyle for the rise. People are now eating more fatty foods and they are also exercising less.
They have warned of a huge rise in obesity-related diseases, including diabetes and heart disease, unless people start to lose weight. Over 30,000 deaths a year are caused by obesity in England alone.
The National Institute for Clinical Excellence approved the use of new anti-obesity drugs on the NHS in 2001.
Experts at the drugs watchdog said the benefits of the drugs outweighed the costs to the health service.