Cannabis use among high school students decreases once a state legalizes recreational marijuana use, a new study has suggested.

Researchers studied data covering around 1.4 million high school pupils across the U.S., based on annual surveys Centers for Disease Control and Prevention between 1993 and 2017.

When a state legalized recreational cannabis use, they discovered an 8% decrease in the number of students that reported using marijuana in the past 30 days. They also noticed a 9% decline in the number of heavy users that said they had used cannabis at least 10 times in the previous 30 days.

The study was compiled by Mark Anderson, Benjamin Hansen and Daniel Lees and published in JAMA Pediatrics today.

“There was no evidence that the legalization of medical marijuana encourages marijuana use among youth,” said the researchers. “Moreover, the estimates reported in the Table showed that marijuana use among youth may actually decline after legalization for recreational purposes.”

Anderson et al pointed to a December 2018 study called Prevalence of Cannabis Use in Youths After Legalization in Washington State to back up their claims. They suggested that adult-use legalization makes it more difficult for teenagers to obtain marijuana as drug dealers are replaced by licensed dispensaries that require proof of age.

A separate research paper published today provided evidence that teens with jobs are more likely to use cannabis than those who are unemployed. The study was conducted by Washington State University’s Assistant Professor Dr. Janessa Graves and used a similar approach, contrasting pre-legalization Healthy Youth Survey data from 2010 with post-legalization data collated in 2016.

The survey is completed by 76,000 teens in the 8th, 10th, and 12th grades each year. Graves’ research found that between 2010 and 2016 marijuana use decreased significantly among working and nonworking 8th and 10th graders, backing up the paper published in JAMA.

But it found a significant increase among working 12th graders over those that did not work, while the amount they smoked increased in line with the amount of hours they worked. Teens working in retail and the service sector were more likely to use cannabis than those doing odd jobs like babysitting, according to the research.