Daily associations between cannabis use and alcohol use among people who use cannabis for both medicinal and nonmedicinal reasons: Substitution or complementarity?

Objective: People who use cannabis for medicinal (versus nonmedicinal) reasons report greater cannabis use and lower alcohol use, which may reflect a cannabis–alcohol substitution effect in this population. However, it is unclear whether cannabis is used as a substitute or complement to alcohol at the day level among people who use cannabis for both medicinal and nonmedicinal reasons. This study used ecological momentary assessment (EMA) to examine this question. Method: Participants (N = 66; 53.1% men; mean age 33 years) completed daily surveys assessing previous-day reasons for cannabis use (medicinal versus nonmedicinal), cannabis consumption (both number of different types of cannabis used and grams of cannabis flower used), and number of standard drinks consumed. Results: Multilevel models revealed that, in general, greater cannabis consumption on a given day was associated with greater same-day alcohol use. Further, days during which cannabis was used for medicinal (versus exclusively nonmedicinal) reasons were associated with reduced consumption of both cannabis and alcohol. Finally, the day-level association between medicinal reasons for cannabis use and lower alcohol consumption was mediated by using fewer grams of cannabis on medicinal cannabis use days. Conclusions: Day-level cannabis-alcohol associations may be complementary rather than substitutive among people who use cannabis for both medicinal and nonmedicinal reasons, and lower (rather than greater) cannabis consumption on medicinal use days may explain the link between medicinal reasons for cannabis use and reduced alcohol use. Still, these individuals may use greater amounts of both cannabis and alcohol when using cannabis for exclusively nonmedicinal reasons.