BBC Health Correspondent Richard Hannaford: "Xenical has proved successful in trials"

Xenical, the brand name for orlistat, is being launched in the UK on Monday.

Manufactured by Roche Products, it is designed to be used by the more than 15% of the UK population with a clinical obesity problem.

GPs fear they may face a high demand for Xenical since it is the first fat-blocking slimming drug to get the go-ahead from European regulatory authorities.

They want the government to issue national guidance on who should get the drug as it is planning to do with anti-impotence drug Viagra.

The British Medical Association says the NHS budget has been set for the next three years so the government should make decisions about priorities where new drugs are concerned.

And it called for information on the side effects and usage of Xenical to be made available to doctors.

Only people with a BMI of 28 or higher should be offered the drug

NHS finances

Dr Nigel Sylvester, a Winchester GP and chairman of the local Primary Care Group said the arrival of Xenical would have a major impact on National Health Service (NHS) finances.

"This drug should be available to a very small, well defined group of people on the NHS," he said.

"But there are an awful lot of people who just want to eat excessively, who don't like their own shapes, who will also want the drug, and it's going to be very difficult for GPs to decline them."

Dr Nigel Sylvester: "It's going to be very popular"

"We need very clear criteria in the NHS about which patients are going to benefit from this, and how much we're allowed to spend on it."

Dr Sylvester said no new money was being provided by the government to pay for the drug. It would leave GPs in the difficult position of having to prioritise healthcare, he complained.

Who gets the drug?

Roche Products said treatment would cost £1.50 per patient per day. Dr Trish Campbell, the company's medical advisor on Xenical, said the drug would not be on general sale in chemists.

Dr Trish Campbell: Comparisons with Viagra are invalid

"It's purely a prescribable product. It can only be prescribed by doctors," she said. "The licence is very clear that it is a treatment for obesity."

She said comparisons with Viagra were invalid because Xenical would only available to a very controlled group of people.

Roche Products has advised doctors that only people who have been shown to lose at least 2.5kg over a four-week period on a diet alone should get the drug.

Patient benefits

The Obesity Resource Information Centre (ORIC) said some patients lose more weight than others on Xenical.

Mike Thompson talks to people who have tried the new drug

"It is important to carefully identify patients who are most likely to benefit from treatment and also to discontinue therapy in patients who do not lose weight," it said.

Although it welcomes the drug as a "new option" for the obese, it warns it is not a miracle cure and should only be used as part of a weight management programme.

It said over half the weight loss achieved by taking Xenical is due to dietary changes.

There is also no information available on whether it works or is safe if it is taken for over two years.

ORIC calls for careful monitoring of anyone taking Xenical.

Low fat

Xenical works by blocking the absorption of fat in the gut and should be taken as part of a low-fat diet.

People who eat fatty food on Xenical can get stomach ache

If people taking it eat a fatty diet, they experience a bloated and painful stomach and oily diarrhoea.

Patients given Xenical lose an average of 10% in body weight over a year, compared with a 6% loss for those who only follow a low-fat diet.

And they are half as likely to put weight back on.

Although over 50% of British people are overweight, only around 15% are clinically obese, although this number is growing fast.

These are people whose weight in kilos divided by their height in metres squared is over 30. Xenical is likely to be available to people with a BMI of over 28, provided they have other associated risk factors such as diabetes.

Three pills a day

Colin Christie, a retired head teacher from Aberdeen, took part in Xenical trials. When he started taking the drug, he was 162 kg.

Colin Christie:"It worked for me"

The regime involved taking a pill three times a day for three months on top of a controlled diet.

"I lost 32 kilos," he said. "I'm very pleased. It certainly worked for me, but it's not only the drug.

"The eating habits have to be controlled as well. It's a slow operation. I lost a kilo a week, so it's no use for cosmetic dieting."

Like may other Xenical users, Mr Christie did experience some loose bowel problems. "Obviously, without being to exact about it, the body has to get rid of the fat if it is not going to absorb it. But other than first thing in the morning, that was no problem."