BACKGROUND: Adolescent marijuana prevalence has not increased since 2005 despite a substantial decrease in the percentage of adolescents who believe marijuana use leads to great risk of harm. This finding calls into question the long-standing, inverse connection between marijuana prevalence and perceived risk of use, a connection central to many arguments opposing marijuana legalization. We tested 2 hypotheses for why marijuana prevalence did not increase after 2005: (1) decreases in adolescent use of cigarettes and alcohol reduced risk for marijuana use and counteracted the expected risk in marijuana prevalence, and/or (2) perceived risk of harm now plays a smaller role in marijuana use.
METHODS: Data came from the annual, nationally-representative Monitoring the Future study from 1991 to 2016, in which 1 100 000 US students in eighth, 10th, and 12th grade were surveyed.
RESULTS: The entire sample was stratified into 3 mutually exclusive and exhaustive groups on the basis of cigarette and alcohol use. Within each of the 3 groups, marijuana prevalence increased from 2005 to 2016. Paradoxically, when the 3 groups were combined into 1 analysis pool, overall marijuana prevalence did not increase. The seeming paradox results from a decline in the percentage of adolescents who used cigarettes; as this group grew smaller, so too did its disproportionately large contribution to overall marijuana prevalence. Perceived risk of harm from marijuana remained a strong indicator of use throughout 2005 to 2016.
CONCLUSIONS: Perceived risk of marijuana remains tightly associated with use, and adolescent marijuana prevalence today would be at or near record highs if cigarette use had not declined since 2005, according to study projections.