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Marijuana use in adolescents causes psychosis, according to a paper that appeared in the British Medical Journal (BMJ).

This piece of research is significant in that it addresses a key question: in adolescents, is the higher-than-average instances of psychosis with marijuana users a cause-and-effect relationship or simply one of correlation?

In other words, do adolescents who smoke marijuana happen to be the same people who have higher-than-average instances of psychosis or does marijuana actually cause it? Moreover, is the correlation explained by marijuana users who already had psychotic disorders and use marijuana to self-medicate?

According to this latest study -- which makes adjustments for factors like age, sex, socioeconomic status, the use of other drugs, urban versus rural residence, and childhood trauma -- marijuana use “precede the onset of psychotic symptoms.”

Furthermore, the longer an adolescent uses it, the greater the risk for psychotic disorder.

The key assumption made is that the higher-than-average psychosis in individuals who have no prior history of psychosis before marijuana use must mean that marijuana use is causing the psychosis.

One explanation given is that marijuana use exacerbates existing psychotic conditions. Therefore, cases of mild adolescent psychosis that normally would have remained “transitory” could have been made more lasting due to marijuana exposure.

The study, complete over 10 years, had complete data for 1,923 individuals in Germany.

“Regular cannabis use in adolescence predicts poorer educational outcomes, increased risk of using other illicit drugs, increased risk of depression and poorer social relationships in early adulthood,” added two professors in a related editorial in the BMJ.