Examining Daily Associations Between Cannabis Use and Alcohol Use Among People Who Use Cannabis for Both Medicinal and Nonmedicinal Reasons: Substitution or Complementarity?

People who use cannabis for medicinal reasons tend to report elevated cannabis use and reduced alcohol use, which may reflect a cannabis–alcohol substitution effect. However, it is currently unclear whether cannabis is used as a substitute for or complement to alcohol at the day level among individuals who use cannabis for both medicinal and nonmedicinal reasons. This study used ecological momentary assessment (EMA) to examine day-level cannabis-alcohol associations linked to day-level variation in medicinal (versus nonmedicinal) reasons for cannabis use. People reporting cannabis use for both medicinal and nonmedicinal reasons (N=66) completed daily surveys assessing previous-day reasons for cannabis use, cannabis consumption, and alcohol consumption. Multilevel models revealed that days during which cannabis was used for medicinal (versus exclusively nonmedicinal) reasons were associated with reduced consumption of both cannabis and alcohol, and alcohol use was increased on days involving greater cannabis consumption. Further, the day-level association between medicinal (versus exclusively nonmedicinal) reasons for cannabis use and lower alcohol consumption was mediated by fewer grams of cannabis used on those days. Results suggest that day-level cannabis-alcohol associations may be complementary rather than substitutive among people who use cannabis for both medicinal and nonmedicinal reasons, and reduced (rather than increased) cannabis use may explain the link between medicinal reasons for cannabis use and reduced alcohol use. These individuals may still be at risk for cannabis-alcohol co-use-related harms, especially on days when they use cannabis for nonmedicinal reasons.