College students who use large amounts of marijuana and alcohol might be high, but their grades are not, a new study shows.

The grades of those who drank moderately or heavily but smoked little pot did not suffer as much as students who added heavy marijuana use to their drinking, according to study published March 8 in the journal PLOS ONE.

Researchers at Yale and the Institute of Living in Hartford followed 1,100 Connecticut college freshmen over four semesters. The students answered questions online about their monthly alcohol and marijuana use. The researchers found that the students fell into three clusters — those who drank little or no alcohol and smoked little or no pot, those who drank moderately or heavily but used marijuana infrequently, and those who used both substances moderately or heavily. (The researchers found very few freshman who smoked marijuana but did not drink.)

There was little effect on grades in the first two clusters, but the grades of those who drank and smoked pot suffered the most.

“Doing a lot of both drugs had a significant impact on lower grades in our study, and in other studies, with the number of leaves of absences and dropping out of school,” said Godfrey Pearlson, professor of psychiatry and neurobiology, and senior author of the study.

On the other hand, grades of individuals who moderated their use of alcohol and marijuana saw their GPA improve in subsequent semesters.

The first author of the study was Shashwath Meda from the Institute of Living.

Primary funding for the research came from the National Institutes of Health.