Marijuana initiation among American youth and its risks as dynamic processes: prospective findings from a national longitudinal study.

The aim of this study was threefold: (1) to prospectively estimate population incidence rates of marijuana use from late childhood to adolescence, (2) to identify important risk factors, and (3) to examine and demonstrate the dynamic nature of risk factors of marijuana initiation, that is, the degree to which influences change as youth age. The longitudinal data from seven nationally representative age cohorts (aged 10-16 years) of marijuana never-users (N = 4,607) and their parents were used. These data were collected during 1999-2004 under the National Institute on Drug Abuse (NIDA) contract N01DA-8-5063, USA, using the National Survey of Parents and Youth. This survey was designed in part to measure changes in drug-related beliefs, attitudes, and behaviors in American youth and their parents. A series of lagged logistic regression analyses were performed with a cohort-sequential design. Multiple imputation was used to handle missing data and longitudinal replicate weights were incorporated into the analyses. Results showed that the population incidence rates of marijuana use increased from 1.30% to 16.29% from age 11 to 16 years and then appeared to level off. A sharp increase was found during ages 13-15 years. Among six identified important factors, alcohol and/or tobacco use and marijuana offers appeared to be the most important risk factors across ages and age cohorts. Consistent with hypotheses, parental influence and peer influence varied as youth age. Both parental influence and peer influence had significant effects during early adolescence and peer influence continued to middle adolescence. Parental monitoring functioned as a protective factor against peer influence on marijuana initiation, but the effect vanishes during late adolescence. Results provide some empirical evidence of a shift from parental influence to peer influence. The study's limitations are noted.