P aying people to stop sm oking is a very effective method of getting them to quit, a large review of studies has found.

The meta-analysis, in the Cochrane Reviews, covered 33 trials and included more than 21,000 people. The studies were carried out in various settings — primary care clinics, universities, cancer treatment centers and others. All followed the participants for at least six months, checking breath or body fluids for evidence of smoking.

Some trials were better designed and carried out than others, but even taking the weakest studies out of the analysis, the results were the same: People receiving financial rewards were more likely to quit than controls receiving no payments. Importantly, they were also more likely to remain smoke free.

The amount of the reward was irrelevant. There was no difference in effectiveness between trials that paid less than $100 and those that paid more than $700.