Substitution and Complementarity of Alcohol and Cannabis: A Review of the Literature

Background: Whether alcohol and cannabis are used as substitutes or complements remains debated, and findings across various disciplines have not been synthesized to date. Objective: This article is a first step towards organizing the interdisciplinary literature on alcohol and cannabis substitution and complementarity. Method: Electronic searches were performed using PubMed and ISI Web of Knowledge. Behavioral studies of humans with alcohol (or ethanol) and cannabis (or marijuana) and complement(*) (or substitut(*)) in the title or as a keyword were considered. Studies were organized according to sample characteristics (youth, general population, clinical and community-based). These groups were not set a priori, but were informed by the literature review process. Results: Of the 39 studies reviewed, 16 support substitution, ten support complementarity, 12 support neither and one supports both. Results from studies of youth suggest that youth may reduce alcohol in more liberal cannabis environments (substitute), but reduce cannabis in more stringent alcohol environments (complement). Results from the general population suggest that substitution of cannabis for alcohol may occur under more lenient cannabis policies, though cannabis-related laws may affect alcohol use differently across genders and racial groups. Conclusions: Alcohol and cannabis act as both substitutes and complements. Policies aimed at one substance may inadvertently affect consumption of other substances. Future studies should collect fine-grained longitudinal, prospective data from the general population and subgroups of interest, especially in locations likely to legalize cannabis.