The more years that a teenager smokes marijuana, the more likely he or she is to develop psychosis, according to a study published in the March issue of the Archives of General Psychiatry.

Several studies have linked pot smoking to psychosis, and at least one didn’t find a link. Previous studies, however, have not been able to account for factors that might have skewed the results.

To determine the complex relationship between the length of time that a teen smoked pot and the likelihood of developing psychosis, John McGrath, MD, PhD, from the University of Queensland Brain Institute in Australia and his colleagues examined data from 3,801 young adults born between 1981 and 1984.

In teens who’d been smoking marijuana for six years or more, there was a strong relationship between pot smoking and psychosis of various types. A secondary analysis looking only at 228 people with a sibling in the study also found a connection between marijuana and psychosis. The sibling analysis helps rule out factors such as genetics that could have contributed to the study results.

“The nature of the relationship between psychosis and cannabis use is by no means simple,” the authors write. “This demonstrates the complexity of the relationship: Those individuals who were vulnerable to psychosis (i.e., those who had isolated psychotic symptoms) were more likely to commence cannabis use, which could then subsequently contribute to an increased risk of conversion to a non-affective psychotic disorder.”