London: Pharmaceutical diet pills which go on sale in the UK for the first time later this week are no substitute for a healthy lifestyle, according to medical experts.

One of the drugs, Alli, can help those to take it lose 3lb a week, according to its manufacturer GlaxoSmithKline. This is more than three stones over four months.

The drug, is a milder version of prescription-only Xenical, works by reducing the body’s ability to process fat by about 25 per cent. The fat passes straight through the body, creating a need to go to the toilet frequently.

Professor Gareth Williams, professor of medicine at the University of Bristol and the author of Obesity: Science To Practice, said in the British Medical Journal that the side-effects are so severe that “possibly few users will even finish their first pack of Alli, let alone buy a second”.

More seriously, he said: “The drug may cause only a small and transient downward blip in the otherwise inexorable climb in weight.

“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 efforts to promote healthy living, which is the only long-term escape from obesity.”

He said that real-life weight loss may not be as dramatic as in clinical trials.

“Dieters in these trials are highly motivated and under medical supervision,” he said. “People … taking it without medical supervision may achieve an average daily energy deficit of only 100kcal – equivalent to leaving a few French fries on a plate, eating an apple instead of ice cream, or (depending on enthusiasm and fitness) having 10 to 20 minutes of sex.”

The second drug is Appesat, which claims to cause weight loss of just under 2lb per week. It is a seaweed extract, which swells in the stomach and tricks the user into feeling that they are full.

Its long-term benefits were even questioned by Dr Jason Halford, the director of the Study of Human Ingestive Behaviour University of Liverpool, who is paid to advise the drug’s manufacturer.

“The cure for obesity and being overweight will never be found in a pill, packet or a wonder drug,” Dr Halford said. “That can only come from enormous changes to our food and physical environment, which are going to take a long time to achieve.

“Drugs don’t necessarily deal with reasons why people become obese, which are largely psychological.”

About two-thirds of adults and a third of children are obese, according to the Health Survey for England. Last year the number of prescriptions for “fat pills” rose 16 per cent to 1.23 million.

Last week a BBC television investigation by Professor Lesley Regan of St Mary’s Hospital in London found that women taking a placebo starch pill, who thought they were taking a diet pill, lost up to half a stone in six weeks.