The drug, Xenical, works by blocking the enzymes that digest fat which prevents it being absorbed from the gut. With the correct dose, a third of the fat eaten is blocked and excreted instead of ending up on the hips and thighs.
Traditional diet pills target the central nervous system and have an amphetamine-like action which suppresses the appetite. However, they can only be taken for limited periods and trials have shown that patients tend to regain weight lost when they stop using them.
Trials of Xenical, whose chemical name is orlistat, in 4000 patients in the US and Europe have shown it can cut about 600 calories a day. The average patient in the one year trial lost 10 kilos (more than 1.5 stones).
However, it is not a licence to gorge on cream cakes and burgers as the drug has unpleasant side effects . The presence of extra fat in the gut, which must be excreted, causes diarrhoea and a delicate condition known as "anal leakage" leading to "oily faecal spotting" which gets worse as more fat is consumed.
Although some patients dropped out of the trials because of the side effects, one of the most surprising findings was that others welcomed them because they provided a measure of biofeedback, indicating when they were eating too much fat.
On an average British diet containing 90 grams of fat a day, the drug prevents 30 grams being absorbed, equivalent to 270 calories.Doctors say a cut of a third in fat intake could reduce the incidence of heart disease and other disorders.
The drug's producer, Hoffman La Roche, has applied for a licence to the Medicines Control Agency in the UK and is awaiting approval from the Food and Drugs Administration in the US.