A long-term study that followed 408 seventh-graders for over 20 years found no association between marijuana use at a young age and an increased risk of psychosis, depression, and anxiety in adulthood. The study, published in the journal Psychology of Addictive Behaviors, was led by Jordan Bechtold of the University of Pittsburgh School of Medicine.

“There were no differences in any of the mental or physical health outcomes that we measured, regardless of the amount or frequency of marijuana used during adolescence,” Bechtold said in a statement to the American Psychological Association.

The researchers used data from the Pittsburgh Youth Study, which tracked a cohort of boys from seventh-grade into adulthood. After being randomly selected from the Pittsburgh public schools in 1987, the participants were interviewed every six months for two and half years until they were roughly 16 years old. Researchers then interviewed the participants annually for the next ten years and then again, a decade later, when they were about 36 years old.

Of the 408 participants who completed the study 46% were identified as low or non-users of marijuana and the rest were divided between adolescence-limited users (10.7%), late increasing users (21%), and early onset chronic users (22%). The study found no significant differences in the likelihood of mental health problems between these groups.

The researchers found it “particularly striking” that there was no difference in the risk of mental health problems between non-users and those in the early onset chronic group who “were using marijuana (on average) once per week by late adolescence and continued using marijuana approximately 3– 4 times a week from age 20 to 26 years.”

The study also investigated how marijuana use may differ in its impact on long-term mental health between African American and Caucasian men. Previous research found that African-American men are “more likely to have health problems and less likely to have access to quality health care services than white men.” Bechtold and his collaborators studied whether marijuana use might compound these existing inequalities, leading to a greater prevalence of mental health issues, but found that ethnicity did not significantly impact their results.

With political debates about the legalization of marijuana intensifying over the past year, interest in the potential mental health effects of chronic use has increased. In light of previous research, which has suggested that chronic marijuana use may be tied to deleterious effects, such as early onset psychosis and an increased risk of developing bipolar disorder, Bechtold described the study’s findings as “a little surprising” but cautioned that marijuana legalization is “a very complicated issue and one study should not be taken in isolation.”