A jar of cannabis at a Good Meds medical cannabis center in Lakewood, Colo. (Photo by Matthew Staver/For The Washington Post)

Alcohol seems to act as a substitute for marijuana when people hit the minimum legal drinking age, a recent study found.

The study, published in the Journal of Health Economics, found — perhaps not surprisingly — that alcohol consumption spikes among people just over the age of 21. But it also found that marijuana use experienced a substantial drop at the same age.

The finding is important to the debate about public policy to regulate the substances. As economic substitutes, how strictly marijuana laws are enforced would affect alcohol use — and vice versa — because people will tend to use whichever substance is cheapest or more available.

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Previous studies found murky evidence about whether the two act as substitutes or complements. This study, which looked at five years' worth of survey data from the National Survey on Drug Use and Health, looked directly at the drinking age as a threshold for comparison between the months before and after people's 21st birthdays.

It found the probability of having consumed alcohol in the past 30 days increases by about 16 percent just as young people have legal access, whereas marijuana decreases by 10 percent at the same time — indicating that it is, in fact, a substitute.

The phenomenon is also more pronounced among women, according to the study. Although men had higher rates of alcohol and marijuana usage, the frequency of marijuana use decreased 7.5 percent at the drinking age for men, compared to a decrease of 15 percent for women.

For Ben Crost, a co-author of the study and a professor at the University of Illinois, the results of the study means there's a trade-off in what illicit substance policy can be enforced.

[Alcohol use, binge drinking continues to fall among underage]

"If you think alcohol is much more harmful to people's health, then you should probably restrict alcohol use," Crost said in a statement. "If you think marijuana is more harmful, then you might want to consider loosening the restrictions for alcohol."

Public opinion polls have shown that Americans view alcohol as far more dangerous than marijuana, by a nearly 5-to-1 margin. Studies have padded this opinion, showing that alcohol poses a greater danger both to users and to the public.

The opinion is not universally held, though, as some, including U.S. Attorney General Loretta Lynch, have expressed disagreement.

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