At one point in your life, you were probably under 21. Maybe you still are, which is fine. But leading up to that magical moment when the clock struck midnight and you were legally able to acquire and imbibe alcohol, you probably weren’t doing a lot of drinking. (If you were, well, all right then.) What you might have been doing instead was smoking pot.

A new study shows that something drastic happened the second people change from 20 years and 11 months to 21 years old: They stopped smoking pot and started drinking alcohol.

University of Illinois economist Ben Crost wanted to see the correlation between alcohol and marijuana consumption of people between the ages of 18 and 24. What he originally found was rather uninteresting: When people turned 21, the legal drinking age in the U.S., they started drinking a lot more. He was surprised to find that during that same time period, marijuana consumption dropped drastically.

"Alcohol appears to be a substitute for marijuana. This sudden decrease in the use of marijuana is because they suddenly have easy access to alcohol," Crost said.

Using five years’ worth of data from the National Survey on Drug Use and Health that asked participants how many days in the past 30 they’d used alcohol or marijuana, Crost and colleague Santiago Guerrero determined that there was a stark difference between the people who hadn’t yet crossed the 21-year-old threshold and those that had just passed it.

"Whenever there is a discontinuous threshold where something changes, it provides a way to identify a causal effect," Crost said. "You can compare people right above and right below the threshold. They should be very similar in all other respects, except for that one difference.”

Crost and Guerrero also studied men and women separately, to see which gender was more affected by the sudden change in their ability to get alcohol. They found that the change was greater in women than in men. Women’s frequency in marijuana use dropped 15 percent after turning 21, while men’s frequency only dropped seven percent.

If anything, the study best proves that the regulation of both substances is something that needs to be continually discussed, especially if one considers that marijuana could be a possible replacement for alcohol until the legal drinking age is reached.

"We need to take this possible substitution behavior into account," Crost said. "Marginally lowering the minimum legal drinking age would decrease the probability of marijuana consumption in young adults by about 10 percent. So, policies aimed at restricting alcohol consumption among young adults are likely to have the unintended consequence of increasing the use of illegal drugs, such as marijuana."

Source: Crost B, Guerrero S. The effect of alcohol availability on marijuana use: Evidence from the minimum legal drinking age. Journal of Health Economics. 2015.