The first NHS scheme to offer cash rewards for losing weight has helped more than 100 obese people shed nearly two stone each in a year.
Cash payments of up to £425 were offered by an NHS primary care trust in Kent to 402 volunteers who signed up for the year-long "Pounds for pounds" trial in January 2009.
Among the 100 who completed the course, the average weight loss over the year was 25lb. However, they represented only a quarter of the total. In all, two-thirds of the volunteers failed to reach their weight loss target.
The mixed results are a disappointment to advocates of the use of financial incentives to change unhealthy behaviour. A growing body of experts say cash rewards may offer the best hope of persuading people to alter their lifestyles and head off the epidemic of chronic disease associated with smoking, drinking and overeating which threatens Western nations.
Last month, the National Institute for Clinical Excellence held a two-day hearing to discuss the use of cash incentives in health. In addition to helping people lose weight and give up smoking, other schemes have offered rewards to addicts to stay off drugs and £10 record vouchers to young people who agree to have a test for chlamydia.
An independent evaluation of the Kent weight loss scheme by the University of Sheffield concluded that financial incentives worked for some people, but the high drop-out rate meant the true impact was unclear.
Claire Martin, acting assistant director of public health for NHS Eastern and Coastal Kent PCT, said: "Very often people lose weight, but when they stop their diet the weight returns. We need to invest in programmes that return a sustained weight loss and produce long-term health benefits.
"There were high drop-out rates and so it is very difficult to interpret the results to show how successful this would be across our population."
The cost of the Kent scheme, run by a private company, Weight Wins, is around £180 per head. A spokeswoman for the Department of Health said the cost to the NHS of treating obesity-related conditions was £4.2bn.
Winton Rossiter, chief executive of Weight Wins, said 745 people had joined the scheme, more than half through the NHS Kent scheme and the remainder as private customers, paying a monthly fee. The average weight loss after one year was 15.8lb.
"These results suggest that long-term financial incentives could be the best single weapon in the war on obesity," he said. Weight Wins is now offering a maximum payout of £3,000 to private customers who lose 150lb over 21 months and keep it off for three months. Customers pay £135 to join the scheme and £30 a month.
* In Essex, pregnant women who smoke have been offered up to £60 in food vouchers if they give up.
* In Hammersmith, west London, and in Bournemouth, the NHS has offered those under the age of 25 a £10 HMV voucher to have a chlamydia test.
* Weight Wins, a private company, charges a £10-£30 monthly fee and offers £150 to clients who lose 50lb and up to £3,000 for people who lose 150lb.
* In the US, employees of General Electric were paid up to $750 (£500) if they gave up smoking for at least 12 months.
* Several companies in the US offer a similar service, including Virgin Health Miles, StickK and HealthyWage.