Perceived friends' use as a risk factor for marijuana use across young adulthood.

Perceived social norms of substance use are commonly identified as a risk factor for use. How the strength of association between perceived friends' use and substance use may change across development has not yet been documented. The current analysis considers how the associations between perceived friends' marijuana use and participants' own use of any marijuana in the past year changes from ages 18 to 30 using longitudinal data from the United States national Monitoring the Future study from 1976 to 2014 (N = 30,794 people). Time-varying effect modeling (TVEM) was used to examine the associations between perceived friends' use of marijuana and participants' own annual marijuana use by age, as well as the extent to which these time-varying associations were moderated by sex, race/ethnicity, and parental education. Associations between perceived friends' use and own marijuana use increased with age. In addition, the association between perceived friends' use and own marijuana use significantly varied by demographic groups, such that it was significantly greater for men from ages 19 to 24 and from ages 27 to 30, compared with women; for Whites, compared with other race/ethnicities, across all ages; and for individuals whose parents attended college, compared with those whose parents had a high school education or less, across all ages. Results suggest that perceived friends' marijuana use becomes an even more important marker for increased marijuana use as people age through young adulthood. Therefore, the role of peers in substance use remains crucial beyond adolescence and should be incorporated into intervention strategies for young adults.