Reductions in smoking progression from 10th to 12th grade across 5 cohorts in Southern California 1995-2014.

INTRODUCTION: While initiation of adolescent cigarette smoking has declined in the last two decades, changes to smoking progression have not been examined. METHODS: Survey data were from five cohorts of youth in Southern California (grade 12 in 1995, 1998, 2001, 2004, or 2014; n=2346). Among those who had smoked a cigarette by grade 10 (n=418), we modeled self-reported past-month, past-year, and lifetime number of cigarettes smoked in grade 12 as a function of grade 10 number of cigarettes and cohort (1995, 1998, 2001, 2004 vs 2014). RESULTS: Smoking progression declined markedly over time. Among youth who had smoked by grade 10, the 1995 cohort smoked on average 23 times as many lifetime cigarettes by grade 12 as the 2014 cohort (adjusted rate ratio, ARR=22.9; 95% CI: 9.40-55.6). Similar findings were observed for the past year (ARR=30.1; 95% CI: 6.75-134) and the past month (ARR=12.1; 95% CI: 2.19-67.0) number of cigarettes smoked. Over time, females smoked less, but the pattern of decreasing progression in lifetime cigarettes smoked was more pronounced for females than males (lifetime, p for interaction <0.0001). CONCLUSIONS: The rate of progression among those initiating cigarette smoking in early high school decreased markedly over the two-decade period starting in 1995, suggesting that social or environmental factors may play a substantial role in the escalation of smoking through adolescence.