Rimonabant, aka Acomplia, HOT NEW OBESITY PILL

But NHS chiefs warned people not to expect it to become widely available straight away as the cost-effectiveness of the pill needed to be assessed.

Rimonabant is the first drug to target factors governing the body's appetite, metabolism and energy use. Trials showed it can reduce weight by a tenth.

UK experts said it could not replace healthy food and regular exercise.

In the UK, it is estimated that one in five men and a quarter of women are obese.

Cost

But at a cost of over £55 for a month's treatment, it could end up costing the NHS billions of pounds of money.

The drug still has to be assessed by NHS advisers the National Institute for Health and Clinical Excellence.

The review is not expected for another two years and NHS bosses warned the public not to expect its widespread use immediately.

Gill Morgan, chief executive of the NHS Confederation, said primary care trusts may be reluctant to prescribe the drug ahead of NICE guidance.

"PCTs receive a fixed allocation of money to deliver all the services for their local community and have to take difficult decisions on competing priorities."

And she added weight loss was "predominantly about diet and exercise".

Preventative measures aren't enough on their own Dr David Haslam, of the National Obesity Forum

The drug's manufacturer, Sanofi Aventis, has argued the drug represents good value for money when set against the £7bn per year cost of tacking obesity.

Other anti-obesity drugs are already available, but rimonabant is the only one to target the endocannabinoid system, which governs the body's appetite.

In a series of trials involving more than 6,000 patients in the US and Europe, a quarter lost more than 10% of their initial body weight after a year. About half lost more than 5%.

Side effects in some patients included nausea, dizziness and anxiety.

Significant improvements in measures of glucose control, cholesterol and triglyceride blood fats were also seen.

Smoking

Rimonabant, sold under the brand name Acomplia, is currently licensed for the treatment of obese patients, or overweight patients with associated risk factors such as type 2 diabetes or poor cholesterol and triglyceride readings.

But there are also trial data suggesting that the drug can help people give up smoking by overcoming their cravings.

However at present it is only being marketed to tackle obesity.

Dr David Haslam, clinical director of the National Obesity Forum, said the drug offered a real opportunity to get to grips with the obesity crisis.

"Preventative measures aren't enough on their own. It brings into focus the priority that should be given to treatment as well as prevention.

"The launch of rimonabant is important news for patients who are overweight, with type 2 diabetes, or low HDL cholesterol or high triglycerides."