The decline in adolescent tobacco and nicotine product use has now ceased, therefore more attention is required on prevention, in which supporting tobacco refusal selfefficacy holds great potential. This study aimed to develop an intervention to support early adolescent tobacco refusal self-efficacy and to evaluate the effectiveness of the intervention among early adolescents in comprehensive school grades 4 to 6. The study was conducted in two phases.

The development phase included three sub-studies strengthening the theoretical basis for the intervention and further developing a previously developed digital health game intervention. 1) A qualitative descriptive study described self-efficacy in peer interactions among adolescents (n = 155). 2) A systematic review summarised evidence on digital interventions to support refusal self-efficacy in child and adolescent health promotion. 3) A cross-sectional study explored factors associated with tobacco refusal self-efficacy among early adolescents (n = 295). The evaluation phase consisted of a two-arm cluster randomised controlled trial to examine the effectiveness of the intervention in schools at three measurement points on tobacco refusal self-efficacy among early adolescents (n = 781).

This study found that adolescents’ have an active role in the formation of their self-efficacy, and it is also influenced by their social atmosphere. Tobacco refusal self-efficacy in early adolescence is especially influenced by the social atmosphere, namely the smoking behaviour of peers and relatives. The evidence supported the use of digital interventions with a sound theoretical basis to promote refusal selfefficacy. The results demonstrated the effectiveness of the intervention on tobacco refusal self-efficacy among 12-year-olds and early adolescents with a smoking friend or parent, and on the sources of tobacco refusal self-efficacy.

Based on the results, early adolescent tobacco refusal self-efficacy can be strengthened through the intervention. The results support the implementation of the intervention into school contexts. In general, a systematic evaluation of existing digital health interventions is needed to support their implementation.