For the study, researchers examined alcohol sales data included in Nielsen's Retail Scanner database, which includes product-level sales data from 90 retail chains across the United States. The researchers say this represents an improvement over other ways of measuring alcohol consumption — survey respondents, for instance, are known to severely lowball their alcohol consumption when asked about it by interviewers.
The researchers compared alcohol sales between states that implemented medical marijuana laws and those that didn't, before and after the change in marijuana laws. They also corrected for a number of economic and demographic variables known to affect alcohol use, such as age, race and income.
“We find that marijuana and alcohol are strong substitutes” for each other, the study concludes. “Counties located in [medical marijuana] states reduced monthly alcohol sales by 15 percent” after the introduction of medical marijuana laws.
If these findings are correct, it's likely that they understate the effect of full marijuana legalization on alcohol use. Under medical marijuana laws, only a small handful of people are legally able to access the drug — patients wishing to use it must typically obtain a recommendation from a doctor, and in most states only certain conditions are eligible for treatment with marijuana. Full recreational legalization, as is the case now in Colorado and seven other states, means that any individual can purchase pot on demand.
While not all of the existing research agrees that marijuana availability decreases alcohol use, a solid body of evidence points to that conclusion. An analysis last year of 39 reports on the subject found that 16 supported the idea that people substitute marijuana use for alcohol, while 10 studies suggested that marijuana availability actually increased alcohol use. Twelve additional studies supported neither conclusion.
Unlike alcohol, marijuana has no known fatal dose — people don't die of marijuana poisoning. Relative to marijuana, alcohol is more addictive, far more likely to cause vehicle accidents, and much more closely linked to violent and aggressive behavior.
In the United States, excessive alcohol use kills nearly 90,000 people each year, according to the Centers for Disease Control and Prevention.