Wellington, Aug 2 - Top-selling weight reduction pill Xenical is under scrutiny about whether it works in the long-term. Massey University's professor of nursing practice, Jenny Carryer, was to present a paper in Palmerston North today questioning the effectiveness of the drug once people stopped taking it.

Xenical works by blocking the absorption of fat and has been one of the most successful prescription drugs ever sold in New Zealand. Its distributor, Roche Products, has sold more than $12 million worth of the drug -- which costs patients $5-7 a day -- in the year since its release. Roche recommends that the pill is combined with healthy eating and increased exercise.

But Dr Carryer said that there had been little research on whether people gained weight after they stopped taking the pills. It was unlikely that people would stay on Xenical forever, given its cost.

Her concern was that obesity was caused by many factors, particularly genetic factors, and those were not necessarily resolved by the use of Xenical. "I'm concerned that, no matter what they do, when they stop taking Xenical their body will want to return to their pre-Xenical weight." Dr Carryer, whose doctorate was on women's experience of body size, has submitted a joint paper, with Otago University senior pharmacy lecturer Dr Kath Ryan, to British journal Social Science And Medicine questioning the effectiveness of the popular drug.

Roche marketing director Glenda Macdonald said doctors prescribed the drug because it reduced the risk of health problems, such as heart disease, associated with obesity. The firm had needed to prove such benefits before it was released on to the market, she said. Ms Macdonald said one study showed that people who stopped using Xenical gained half as much weight in the first year after they stopped taking the medication as people who had dieted but not used the drug.

Xenical put people off eating fat, she said. Another study, which had examined use of the drug for five years in Sweden, was due out soon.