Cannabis and psychosis

The relationship between cannabis use and psychosis has, in recent decades, become a focus of controversy. The National Institute of Mental Health has stated that "research has found increasing evidence of a link between marijuana and schizophrenia symptoms." In a report issued in 2000, the National Academy of Sciences noted that some researchers had proposed a link between cannabis use and schizophrenia, as well as between cannabis use and a unique type of psychosis. They observed that "marijuana use alone—without the influence of additional risk factors—is unlikely to provoke a psychosis that persists longer than intoxication." Likewise, a number of reviews have concluded that cannabis use only results in a significant increase in risk of psychosis when coupled with additional risk factors, in particular, an underlying genetic vulnerability.

There is evidence that the brains of people with psychosis who previously used cannabis differ from those of healthy individuals with respect to areas such as the cingulate and prefrontal cortex. Research has consistently found that people with psychosis have higher rates of cannabis use, and that there exists an association between cannabis use and schizophrenia and other forms of psychosis. Some studies have also concluded that cannabis use is associated with an earlier age at onset of schizophrenia. However, it is less clear whether this association is a result of the specific effects of cannabis use. Proposed mechanisms by which this possible link could occur include the effects of dronabinol, a pure isomer of THC, and its ability to induce "acute psychotic states," which, in turn, have been called "the early signs of schizophrenia and related disorders." Further complicating the establishment of causation, another cannabinoid found in cannabis, cannabidiol (CBD), is thought to have antipsychotic properties. CBD has been shown to have anxiolytic properties and inhibit the psychotomimetic effects of THC, yet CBD content varies widely and has rarely been accounted for in epidemiological studies. It is believed that cannabis' effects on the risk of schizophrenia is especially pronounced among those who begin using the drug early.

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