OBJECTIVES: To assess the risk of becoming a smoker in adulthood associated with parental smoking as well as the smoking of siblings and close friends. DESIGN: A cross-sectional study. SETTING: 4 oral healthcare centres in Finland and a follow-up. PARTICIPANTS: An age cohort born in 1979 (n=2586) and living in four Finnish towns. Of those reached by the 2008 follow-up, 46.9% (n=1020) responded. PRIMARY AND SECONDARY OUTCOME MEASURES: Smoking behaviour at the age of 29. Smoking behaviour at the age of 13 and smoking behaviour of family members and close friends. RESULTS: Smoking of a current close friend was strongly associated with participants' own smoking (OR 5.6, 95% CI 3.6 to 8.8). The smoking of a close friend during schooldays was similarly associated (OR 2.9, 95% CI 1.8 to 4.5). Smoking among men was associated with the smoking behaviour of mothers and siblings while that among females was not. CONCLUSIONS: The impact of a smoker as a close friend is greater than that of a smoking parent or sibling in school age when it comes to smoking behaviour in adulthood. This should be taken into consideration when attempting to prevent smoking initiation or continuation. TRIAL REGISTRATION: At clinicaltrials.gov (NCT01348646).
