Effects of cannabis use on alcohol consumption in a sample of treatment-engaged heavy drinkers in Colorado

Background and aims: Cannabis is commonly used among people who drink alcohol, but evidence suggests a nuanced relationship between alcohol consumption and cannabis use. In particular, among individuals undergoing alcohol treatment, the impact of cannabis on alcohol intake may depend on cannabis use frequency. We aimed to test the effects of within-day cannabis use on total drinks consumed and likelihood of binge drinking on a given day across all participants and compare these relationships between males and females and between individuals who reported infrequent and frequent cannabis use.

Design: This observational study is a sub-study of a larger randomized controlled trial (RCT). Individuals were included from the RCT if they reported any cannabis use and were divided into groups based on cannabis use patterns. Alcohol use was compared within and between groups.

Setting: Individuals were recruited from 2016-2020 from community and university settings in Denver and Boulder, Colorado, USA.

Participants: Of the 182 individuals enrolled in the RCT, N=96 cannabis-using subjects were included in these analyses.

Measurements: Subjects completed a Timeline Followback (TLFB) at baseline, 4 weeks, 8 weeks (end of treatment), and 20 weeks. Daily data on alcohol and cannabis use from the TLFB at all timepoints were analyzed.

Findings: Across the sample (N=96), individuals drank approximately 29% fewer drinks (95% Confidence Interval (C.I.) 18%-39%, p<.001) and were 2.06 times (95% C.I. 1.37-3.08, p<.001) less likely to have a binge-drinking episode on days that cannabis was used compared with days cannabis was not used. These patterns were observed in males, females and the infrequent and frequent cannabis use groups. Findings were inconclusive regarding differences in the association between cannabis use and alcohol outcomes when comparing males and females and when comparing infrequent and frequent cannabis use groups.

Conclusions: Heavy drinkers engaged in treatment to reduce their alcohol consumption who also use cannabis appear to increase their cannabis use on days when they reduce their alcohol consumption.