RESUMEN
Background: As a big European city famous for its party scene, Berlin attracts college students that are a high-risk population for cannabis use and use disorder. College years are often associated with new behavior patterns, but the factors leading to cannabis initiation are rarely studied past adolescence. This study describes the longitudinal evolution of college students' cannabis use over two years and its correlates. Method: Data was collected among all students of Berlin's public colleges via two online surveys (N = 1,201, mean interval = 16 months). Multivariable binary logistic regressions were performed on four outcomes: regular use, use initiation, use reduction and use cessation. Several dimensions of covariates were used: socio-demographic factors, psychological (locus of control, impulsivity, psychiatric diagnosis), behavioral (other substance use), perceived harm, declared intention to reduce and setting of cannabis use. Results: Overall, the majority of respondents did not change their cannabis use. The factors for use initiation (impulsivity, tobacco and alcohol use) were not fully symmetric to the factors leading to cutting down/quitting (locus of control, perceived harm, tobacco use). Perceived harm had an impact on quitting, but not on reducing use. The intention to reduce did not significantly predict subsequent use behavior. Most regular users use cannabis at home, which was associated with a low probability to reduce. Conclusions: No simple symmetry exists between correlates of initiation and cessation: tobacco co-use is important for both, while impulsivity and alcohol use lead to initiation and internal locus of control facilitates cessation.