Dutch researchers set up a study to figure that out. Researchers surveyed 2,000 teens in the Netherlands through their adolescence. Subjects' were asked about their marijuana use, then asked questions designed to gauge their mental state ("Do you ever see things that others do not?"). The research team also factored in alcohol and tobacco use and a family history of mental illness. If the teens reported mental-health issues years after smoking marijuana, the researchers assumed maybe pot was the cause. If the opposite happened, maybe people with psychosis just smoked more pot.

The results were, disappointingly, inconclusive. In the survey, 44 percent of teens admitted smoking pot. Using it at age 16 was linked to psychotic symptoms at age 19. But the researchers found a link running in the opposite direction, too. Teens showing psychotic symptoms were also more likely to smoke pot later in life, well after the symptoms developed.

So maybe it's not a simple A-to-B conclusion with marijuana use and psychosis; maybe both points have some truth to them - or neither. Earlier studies couldn't definitively link marijuana and psychosis in one direction or the other, and this study couldn't, either.

[The Atlantic]