Cannabis use in young adulthood is common, yet few studies have explored how it predicts changes in psychopathology and functional well-being in community samples. We assessed these links using both self-reported frequency of cannabis use and hair THC concentrations.

Data came from a community sample of young adults (N = 863) who reported cannabis use (weekly-to-daily use: n = 150) and provided hair samples at age 20 (cannabis detected: n = 110). Liquid chromatography–tandem mass spectrometry quantified delta-9-tetrahydrocannabinol (THC) and cannabinol (CBN) concentrations in hair. At ages 20 and 24, participants reported psychopathology (psychotic-like experiences, problematic substance use, internalizing symptoms, and aggression) and functional wellbeing (general well-being, delinquency, and not being in employment, education, or training). Multiple linear and logit regression models tested associations between six different continuous and dichotomous operationalizations of self-reported and objective cannabis exposure at age 20 and psychological and functional well-being at age 24, adjusting for sex, sociodemographic characteristics, and the outcomes measured at age 20.

Both self-reported frequency of cannabis use and hair THC concentrations predicted increases in psychotic-like experiences and internalizing symptoms, increased aggression, decreased general well-being, higher odds of not being in employment, training, or education, and more problematic substance use from age 20 to 24, with small effect sizes. Composite exposure scores derived from self-reports and hair data were not more informative than either source alone.

Frequent cannabis use predicted adverse changes in psychopathological outcomes from ages 20 to 24, regardless of how it was assessed.