Slimming pill that works by reversing 'cannabis munchies'

In clinical trials, volunteers taking the experimental drug taranabant consumed a third fewer calories than those on a dummy pill and lost "significant" weight within 12 weeks.

A larger trial involving thousands of obese patients in the U.S. is under way.

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If the drug is confirmed to be safe, it could be approved for use among clinicallyobese patients in less than two years.

Taranabant is designed to have the opposite effect on the brain as cannabis - a drug which can trigger food cravings known as the "munchies".

When cannabis is smoked, active chemicals in the drug stimulate part of the brain known as cannabinoid receptors, creating the side effects of euphoria, relaxation and hunger pangs.

Taranabant works by blocking these receptors - suppressing appetite and speeding up the body's metabolism.

Dr Steven Heymsfield, of Merck Research Laboratories in New Jersey, said: "The effects of marijuana on appetite have been known for millennia from its medicinal and recreational use.

"The ingredient responsible stimulates cannabinoid receptors. When you block the cannabinoid system with an antagonist like taranabant, you suppress appetite."

In a 12-week trial of 533 obese volunteers, those taking the drug lost significantly more weight than people given a useless placebo pill, the researchers report in the journal Cell Metabolism.

Weight loss varied between 3lb and 9lb.

Further studies showed that the drug patients consumed 27 per cent fewer calories each day than those taking the placebo.

Their metabolism was also more active - and they used more energy while resting and burned up more fat.

Even low doses of the drug helped lose weight, the study found. "That was surprising," said Dr Heymsfield.

"We didn't expect weight loss at all doses."

However, at high doses, the drug had some unpleasant side effects.

Larger amounts led to stomach upsets, nausea and vomiting, and caused patients to become more irritable.

A larger, phase 3, trial of the drug has now started. The first results from this are expected later in the year.

Dr Heymsfield added: "All we have here is 12 weeks - we don't yet know what will happen at six months or a year."

One in five adults in the UK - almost 10million - is clinically obese. Taranabant is the second anti-obesity pill to target the brain's cannabinoid receptors.

In 2006, another slimming aid called rimonabant was launched in the UK.

Available on the NHS for the seriously obese, it is taken by around 10,000 patients and costs £2 per day.

As well as controlling appetite, weight, energy use and metabolism, rimonabant can also combat a smoker's carving for nicotine.

However, there are concerns about the use of drugs to help lose weight.

Last year, a study in The Lancet warned that rimonabant - also known as Acomplia - could increase the risks of mental illness. It found that those taking the drug were 40 per cent more likely to suffer depression.

The U.S. Food and Drug Administration refused to give approval for the drug, following concerns about side effects.

Although some doctors are excited about weight loss drugs, some have failed to live up to their early promise.

Last year, the British Medical Journal said taking them long term would bring only "modest" weight loss - and that many patients would stay fat.

The National Institute for Clinical Excellence - the NHS drugs "rationing" body - recommends stopping the use of a slimming drug if patients fail to lose five per cent of their weight within three months.