Effectiveness and Cost-Effectiveness of Repeated Implementation Intention Formation on Adolescent Smoking Initiation: A Cluster Randomized Controlled Trial

Objective: Forming implementation intentions (if-then plans) about how to refuse cigarette offers plus antismoking messages was tested for reducing adolescent smoking. Method: Cluster randomized controlled trial with schools randomized (1:1) to receive implementation intention intervention and messages targeting not smoking (intervention) or completing homework (control). Adolescents (11-12 years at baseline) formed implementation intentions and read messages on 8 occasions over 4 years meaning masking treatment allocation was not possible. Outcomes were: follow-up (48 months) ever smoking, any smoking in last 30 days, regular smoking, and breath carbon monoxide levels. Analyses excluded baseline ever smokers, controlled for clustering by schools and examined effects of controlling for demographic variables. Economic evaluation (incremental cost effectiveness ratio; ICER) was conducted. Trial is registered (ISRCTN27596806). Results: Schools were randomly allocated (September- October 2012) to intervention (n = 25) or control (n = 23). At follow-up, among 6.155 baseline never smokers from 45 retained schools, ever smoking was significantly lower (RR = 0.83, 95% CI [0.71, 0.97], p = .016) in intervention (29.3%) compared with control (35.8%) and remained so controlling for demographics. Similar patterns observed for any smoking in last 30 days. Less consistent effects were observed for regular smoking and breath carbon monoxide levels. Economic analysis yielded an ICER of $134 per ever smoker avoided at age 15-16 years. Conclusions: This pragmatic trial supports the use of repeated implementation intentions about how to refuse the offer of a cigarette plus antismoking messages as an effective and cost-effective intervention to reduce smoking initiation in adolescents.