Youngsters tend to swap marijuana with alcohol after turning 21

In a new study, scientists have found that when people turn 21, while the consumption of alcohol drastically increases, marijuana use significantly declines.

Ben Crost at the University of Illinois, who conducted the economist research, which looked at marijuana and alcohol use in people between the ages of 18 and 24, said that alcohol appears to be a substitute for marijuana. This sudden decrease in the use of marijuana was because they suddenly had easy access to alcohol.

Crost and Santiago Guerrero used five years of data from the National Survey on Drug Use and Health.

Crost said that there was a need to take this possible substitution behavior into account. 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 were likely to have the unintended consequence of increasing the use of illegal drugs, such as marijuana.

The study also analysed men and women separately. Although men have higher baseline use levels of both alcohol and marijuana, the effect of the minimum legal drinking age is larger for women. For example, the frequency of marijuana use for men decreased 7.5 percent. Women's frequency of use decreased 15 percent.

The study is published in the Journal of Health Economics.