Abstract: Twenty-seven RCTs were included. The interventions were very heterogeneous in the components of the family intervention, the other risk behaviours targeted alongside tobacco, the age of children at baseline and the length of follow-up. Two interventions were tested by two RCTs, one was tested by three RCTs and the remaining 20 distinct interventions were tested only by one RCT. Twenty-three interventions were tested in the USA, two in Europe, one in Australia and one in India. The control conditions fell into two main groups: no intervention or usual care; or school-based interventions provided to all participants. These two groups of studies were considered separately. Most studies had a judgement of 'unclear' for at least one risk of bias criteria, so the quality of evidence was downgraded to moderate. Although there was heterogeneity between studies there was little evidence of statistical heterogeneity in the results. We were unable to extract data from all studies in a format that allowed inclusion in a meta-analysis. There was moderate quality evidence family-based interventions had a positive impact on preventing smoking when compared to a no intervention control. Nine studies (4810 participants) reporting smoking uptake amongst baseline non-smokers could be pooled, but eight studies with about 5000 participants could not be pooled because of insufficient data. The pooled estimate detected a significant reduction in smoking behaviour in the intervention arms (risk ratio [RR] 0.76, 95% confidence interval [CI] 0.68 to 0.84). Most of these studies used intensive interventions. Estimates for the medium and low intensity subgroups were similar but confidence intervals were wide. Two studies in which some of the 4487 participants already had smoking experience at baseline did not detect evidence of effect (RR 1.04, 95% CI 0.93 to 1.17). Eight RCTs compared a combined family plus school intervention to a school intervention only. Of the three studies with data, two RCTS with outcomes for 2301 baseline never smokers detected evidence of an effect (RR 0.85, 95% CI 0.75 to 0.96) and one study with data for 1096 participants not restricted to never users at baseline also detected a benefit (RR 0.60, 95% CI 0.38 to 0.94). The other five studies with about 18,500 participants did not report data in a format allowing meta-analysis. One RCT also compared a family intervention to a school 'good behaviour' intervention and did not detect a difference between the two types of programme (RR 1.05, 95% CI 0.80 to 1.38, n = 388). No studies identified any adverse effects of intervention. There is moderate quality evidence to suggest that family-based interventions can have a positive effect on preventing children and adolescents from starting to smoke. There were more studies of high intensity programmes compared to a control group receiving no intervention, than there were for other compairsons. The evidence is therefore strongest for high intensity programmes used independently of school interventions. Programmes typically addressed family functioning, and were introduced when children were between 11 and 14 years old. Based on this moderate quality evidence a family intervention might reduce uptake or experimentation with smoking by between 16 and 32%. However, these findings should be interpreted cautiously because effect estimates could not include data from all studies. Our interpretation is that the common feature of the effective high intensity interventions was encouraging authoritative parenting (which is usually defined as showing strong interest in and care for the adolescent, often with rule setting). This is different from authoritarian parenting (do as I say) or neglectful or unsupervised parenting.

Plain-language summary: We identified 27 trials; 23 in the USA and one each in Australia, India, the Netherlands, and Norway. The focus varied amongst the studies. Fifteen trials focused on substance use prevention: six focused only on tobacco prevention; one focused on alcohol; one on general substance abuse; three on tobacco, alcohol and marijuana; two on alcohol and tobacco; and two on tobacco and cardiovascular health. Two trials focused on HIV and unsafe sex prevention. Ten trials focused on family functioning, child development and modifying adolescent behaviour. Duration of follow-up after the intervention was very varied, ranging from 6 months to over 15 years for the studies which intervened with mothers of very young children. Nine trials provided data to compare a family tobacco intervention to no intervention on future smoking behaviour for those who did not smoke at the start of the study. We could not include data from a further eight trials. The results showed a significant benefit of family-based interventions over the control comparison on preventing experimentation with or taking up regular smoking. Our estimate suggested that family interventions could reduce the number of adolescents who tried smoking at all by between 16 and 32%. Two trials provided data to compare a combined family plus school intervention to a school intervention and also favoured the family-based intervention. The estimate suggested that the addition of a family intervention might reduce the onset of smoking by between 4 and 25%. We could not include data from a further five trials. Our interpretation is that the common feature of the effective interventions was encouraging authoritative parenting (which is usually defined as showing strong interest in and care for the adolescent, often with rule setting). This is different from authoritarian parenting (do as I say) or neglectful or unsupervised parenting. Because most of the randomised controlled trials included in the review did not report their methods in sufficient detail to be confident that the results were not biased, we judged the quality of the evidence to be moderate, which means that the estimate of effect is uncertain. There is moderate quality evidence that family-based interventions can prevent children and adolescents from starting to smoke. Intensive programs may be more likely to be successful than those of lower intensity. There is also evidence to suggest that adding a family-based component to a school intervention may be effective. As the interventions and settings in the review differed considerably, it is important that family-based programmes continue to be evaluated.