College kids who use marijuana and drink alcohol have lower grade point averages (GPAs) than students who do not experiment with cannabis and alcohol, a study by Yale researchers found. The study, which was released in PLoS ONE journal Wednesday, tracked the monthly alcohol and drug use of 1,142 students over the course of four semesters starting their freshman year of college.

Students’ colleges were not identified in the study, but students did indicate how many days within a month they smoked weed or drank alcohol, as well as the amount of alcohol consumed each time they drank. Students who drank alcohol and smoked marijuana regularly were found to have the lowest GPAs of three different categories, including students who consumed heavy amounts of marijuana and alcohol, those who consumed little to no marijuana or alcohol and students who drank alcohol but rarely used marijuana.

Read: More Americans Want The Feds To Follow State Cannabis Laws

Students who consumed minimal marijuana and alcohol had the highest GPAs, averaging a 3.1 GPA, while students who mostly drank alcohol but didn’t consume cannabis earned an average 3.03 GPA. Students who used both marijuana and alcohol averaged a 2.66 GPA.

Godfrey  Pearlson, professor of psychiatry and neuroscience and lead author of the study, told Inside HigherEd the negative impact of alcohol and marijuana can be explained.

“If you’re hanging out with friends and drinking and smoking marijuana, it’s time you can’t spend studying — that’s one explanation,” he said. “Another explanation is harming yourself cognitively. Your working ability and your ability to concentrate are affected by the drug. Marijuana smoking by itself produces a certain degree of apathy, so you’re less motivated to study. If it’s less important to you, you’re less likely to engage in it.”

Although students who used both substances had the lowest GPAs, when they reduced their intake, they were able to improve their grades over time.

The National Institute on Alcohol Abuse (NIAA) said in a separate report that of the 60 percent of American college students ages 18 to 22 who drank alcohol within the past month, two to three admitted to binge drinking at that same time. NIAA’s 2014 Monitoring the Future report found 4.3 percent of college students smoke marijuana on a daily basis.