People replace marijuana with alcohol at 21

A study of marijuana and alcohol use in people between the ages of 18 and 24 has revealed that marijuana users change over to alcohol at the age of 21.

The study found a drastic increase in alcohol consumption in people just over 21, the minimum legal age to drink in many countries.

At the same age, there was an equally dramatic drop in marijuana use.

"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," said Ben Crost, an economist at the University of Illinois.

The authors used five years of data from the National Survey on Drug Use and Health. The participants were asked how many days in the past 30 that they had alcoholic drinks or used marijuana.

The study by Benjamin Crost and Santiago Guerrero was published in the Journal of Health Economics.

"Whenever there is a discontinuous threshold where something changes, it provides a way to identify a causal effect," Crost said.

"All of the costs and benefits from policies designed to reduce alcohol consumption, such as the minimum legal drinking age or liquor taxes, need to be assessed," Cost added.

"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," Cost said.