Aims: To examine the association between cannabis use by 18 years and problematic cannabis use at 24 years, considering possible mediating and confounding factors.
Design: Ten-year representative prospective study with data from six time-points in adolescence (mean age 14.9-17.4 years) and two in young adulthood (mean age 20.7 and 24.1 years)
Setting: Victoria, Australia.
Participants: Inception cohort of 1943 secondary school students (95.6% response rate), with 1520 (78% of adolescent participants) interviewed in the final wave.
Measurements: Participants reported frequency of cannabis use for the past 6 months at each time-point in adolescence (age 14-17 years). Cannabis exposure was defined as: maximum frequency of use (occasional, weekly, daily), number of waves of use (1 or 2; 3-6) and first wave of use (early use: first waves 1-3). Young adult (24 years) outcomes were: weekly+ cannabis use and DSM-IV cannabis dependence, referred to collectively as problematic use.
Findings: Of those interviewed at age 24 (wave 8), 34% had reported cannabis use in adolescence (waves 1-6), 12% at a level of weekly or more frequent use; 37% of these adolescent cannabis users were using at least weekly at wave 8, with 20% exhibiting dependence. Persistent adolescent cannabis and tobacco use as well as persistent mental health problems were associated strongly with problematic cannabis use at 24 years, after adjustment for potential confounding factors.
Conclusions: Heavy, persistent and early-onset cannabis use were all strongly predictive of later cannabis problems. Even so, occasional use was not free of later problems. Where there was co-occurring tobacco use or persistent mental health problems, risks for later problem cannabis use was higher.