Increasing numbers of young people are smoking in developing and poorer countries. Programmes to prevent them starting to smoke have been delivered in schools over the past 40 years. We wanted to find out if they are effective.

We identified 49 randomised controlled trials (over 140,000 school children) of interventions aiming to prevent children who had never smoked from becoming smokers. At longer than one year, there was a significant effect of the interventions in preventing young people from starting smoking. Programmes that used a social competence approach and those that combined a social competence with a social influence approach were found to be more effective than other programmes. However, at one year or less there was no overall effect, except for programmes which taught young people to be socially competent and to resist social influences.

A smaller group of trials reported on the smoking status of all people in the class, whether or not they smoked at the start of the study. In these trials with follow-up of one year or less there was an overall small but significant effect favouring the controls. This continued after a year; for trials with follow-up longer than one year, those in the intervention groups smoked more than those in the control groups.

When trials at low risk of bias from randomisation, or from losing participants, were examined separately, the conclusions remained the same. Programmes led by adults may be more effective than those led by young people. There is no evidence that delivering extra sessions makes the intervention more effective.