Both the ICD-10 and the DSM-IV operate with a diagnosis of cannabis-induced psychotic disorder. Despite this, the diagnosis is controversial.

A number of studies have shown that cannabis can induce short-lived psychotic symptoms, and that this could depend on factors such as genetic predisposition. To qualify for a diagnosis of ‘cannabis-induced psychosis’, however, the symptoms must last for at least 48 hours according to the ICD-10. This diagnosis has traditionally been regarded as benign, but this is based on a very limited number of studies.

We have conducted two studies based on Danish treatment registers. Firstly, all patients who had received psychiatric treatment for cannabis-induced psychosis were followed for at least 3 years to determine the rate of subsequent treatment for schizophrenia-spectrum disorders. Secondly, familial predisposition for psychiatric disorders among subjects treated for cannabis-induced psychosis and schizophrenia was compared using competing risk analyses.

Schizophrenia spectrum disorders were diagnosed in 44.5% of the 535 subjects included in the first study. New psychotic episodes of any type were diagnosed in 77.2% and 47.1% received a diagnosis of schizophrenia spectrum disorders more than a year after the cannabis-induced psychosis.

Those who develop cannabis-induced psychosis and schizophrenia spectrum disorders both show elevated levels of predisposition for schizophrenia, other psychotic conditions, and other psychiatric disorders in first-degree relatives. In general, the level of predisposition is of similar magnitude in individuals treated for cannabis-induced psychosis and schizophrenia.

Cannabis-induced psychosis could be an early sign of schizophrenia rather than a distinct clinical entity.