Does cannabis substitute or complement alcohol after recreational cannabis legalization in the Washington State? A three-level mixed-effects modeling.

Background: Whether cannabis substitute or complement alcohol remains inconclusive. Little is known about the daily-level associations between cannabis and alcohol use by cannabis user type (medical vs. recreational use) in people who use alcohol and cannabis within a legalized environment.

Methods: Adult participants were from four waves of Washington Panel Survey during 2014-2016, who consumed both cannabis and alcohol in the past six months. Daily measurements of alcohol and cannabis use in the past week were collected at each wave. Our outcome variable was continuous alcoholic drinks, the exposure was any cannabis use. We applied three-level negative binomial models to account for within-person wave-to-wave and between-person variations, deriving pure within-person within-wave associations between cannabis and alcohol use at the daily level. A cross-level interaction between day-level cannabis use and wave-level medical cannabis recommendation investigated the potential differential substitution/complementarity patterns by medical recommendation status.

Results: 259 respondents with 440 person-waves and 3,051 daily observations were included. We found a statistically significant pure Level 1 (within-person daily-level) effect of cannabis use among recreational cannabis users (IRR = 1.37, 95 % CI: 1.05-1.79, p = 0.02), showing a complementary use pattern. We also found a statistically significant cross-level interaction between medical cannabis recommendation and cannabis use at Level 1 (IRR = 0.57, 95 % CI: 0.34-0.96, p = 0.03), indicating that, differently from recreational users, medical cannabis users may have a substitution use pattern.

Conclusions: Cannabis user type may inform co-use patterns. This study suggested recreational cannabis users tended to use alcohol and cannabis in a complementary manner in Washington State following the legalization of recreational use.