Background The prevalence of cigarette smoking among South Korean adolescents has decreased markedly over the past decade, which may indicate a norm shift between generations of adolescents. The present study aims to identify the effect of banning smoking in public places and increasing cigarette prices on current adolescent smoking, and to determine whether these policies additionally resulted in cohort effects.

Methods Repeated cross-sectional survey data, nationally representative of South Korean adolescents, were used. A total of 853 441 adolescents ranging in age from 12 to 18 years (mean age, 15 years) were identified. Models applied were segmented regression model to detect changes in smoking trends and age-period-cohort model to determine the cohort effects on the trends.

Findings Between 2006 and 2017, smoking decreased from 16% to 9% in boys and from 9% to 3% in girls. After a complete ban on smoking in public places, there were significantly negative trends in the prevalence of smoking for both boys (β=−1.1; 95% CI: −1.9 to −0.2) and girls (β=−0.4; 95% CI: −0.6 to −0.1). Immediate decrease among girls was found after cigarette prices increased (β=−0.8; 95% CI: −1.5 to −0.2). For the cohort effect, the risk of smoking decreased with every consecutive year for boys born after 1998 and girls born after 1997.

Conclusions Our results indicate the presence of cohort effects in the reduction of adolescent smoking. The cohort effect was induced by smoke-free legislation. Research on cohort effects, and methods to denormalise tobacco, will contribute to preventing adolescents from ever trying a cigarette.