​Summary: Most U.S. states have legalized cannabis for medical and/or recreational use. In a 6-month prospective observational study, authors examined changes in adult cannabis use patterns and health perceptions following broadened legalization of cannabis use from medical to recreational purposes in California.

Respondents were part of Stanford University’s WELL for Life registry, an online adult cohort concentrated in Northern California. Surveys were administered online in the 10 days prior to state legalization of recreational use (1/1/18) and 1-month (2/1/18–2/15/18) and 6-months (7/1/2018–7/15/18) following the change in state policy. Online surveys assessed self-reported past 30-day cannabis use, exposure to others’ cannabis use, and health perceptions of cannabis use.

Findings: The sample (N = 429, 51% female, 55% non-Hispanic White, age mean = 56 ± 14.6) voted 58% in favor of state legalization of recreational cannabis use, with 26% opposed, and 16% abstained. Cannabis use in the past 30-days significantly increased from pre-legalization (17%) to 1-month post-legalization (21%) and stayed elevated over pre-legalization levels at 6-months post-legalization (20%). Exposure to others’ cannabis use in the past 30 days did not change significantly over time: 41% pre-legalization, 44% 1-month post-legalization, and 42% 6-months post-legalization. Perceptions of health benefits of cannabis use increased from pre-legalization to 6-months post-legalization. Younger adults, those with fewer years of education, and those reporting histories of depression were more likely to report recent cannabis use pre- and post-legalization. Other mental illness was associated with cannabis use at post-legalization only. In a multivariate GEE (generalized estimating equations) adjusted for sociodemographic characteristics and diagnoses, favoring legalization and the interaction of time and positive health perceptions were associated with a greater likelihood of using cannabis.

Legalized recreational cannabis use was associated with greater self-reported past 30-day use post-legalization, and with more positive health perceptions of cannabis use. Future research is needed to examine longer-term perceptions and behavioral patterns following legalization of recreational cannabis use, especially among those with mental illness.