One of the most important issues for people worried about the consequences of marijuana legalization is the extent to which cannabis serves as a substitute for alcohol, which is more dangerous in several significant ways. Recent evidence from Canada and the United States reinforces the hypothesis that people tend to drink less when marijuana is legally available, although the issue is far from settled.

During 2019, the first full year of legalization in Canada, the volume of beer sold there fell by 3 percent, the Financial Post reports. That drop was large compared to the annual declines seen in the previous five years, which averaged 0.3 percent. Vivien Azer, an industry analyst quoted by the Post, said the accelerated slide was probably related to marijuana legalization, and she predicted that the expansion of cannabis products available from legal sources—which as of January included vapes, edibles, and beverages as well as buds—will "perpetuate this trend."

More rigorous evidence on the relationship between marijuana use and drinking comes from a study reported in the March 2020 issue of the journal Addictive Behaviors. Based on nationwide survey data covering a 10-year period, Zoe Alley and two other researchers at Oregon State University found that college students in U.S. states where marijuana had been legalized for recreational use were less likely to report binge drinking than college students in other states, after taking into account pre-existing trends and several potential confounding variables. The difference was statistically significant among students 21 or older, the cutoff for legally purchasing marijuana.

This apparent substitution effect, the authors note, is consistent with earlier studies that found "reductions in alcohol consumption (especially binge drinking in young adults) and alcohol-related traffic accidents" following the legalization of medical marijuana. A causal connection is plausible in those studies if we assume that some ostensibly medical use is actually recreational (or that some drinking is functionally medical), such that cannabis consumption would displace the use of alcohol.

Prior research, Alley et al. note, has found "a pronounced decrease in marijuana use that coincides with marked increases in alcohol use after minors reach the legal drinking age," which suggests that some young people replace marijuana with alcohol once they are legally allowed to drink. The researchers speculate that "legalizing recreational marijuana use may temper this effect, such that college students over the age of 21 who otherwise would have engaged in binge drinking continue using marijuana instead."