There is, however, evidence that large doses of THC can produce an acute psychosis marked by confusion, amnesia, delusions, hallucinations, anxiety and agitation. Such reactions are rare, though, and occur usually after heavy cannabis use or, in some instances, after acute cannabis use by sensitive/vulnerable individuals. But these effects abate rapidly after discontinuing cannabis use. There is little evidence that cannabis alone produces a psychosis that persists after the period of intoxication. A Swedish study found an association between cannabis use and schizophrenia. In this prospective study of 50,000 Swedish conscripts a dose response relation was found between the frequency of cannabis use and the risk of a diagnosis of schizophrenia over the next fifteen years. Although the value of this study has been debated, it has been suggested that cannabis use may exacerbate the symptoms of schizophrenia and a prospective study has found that continued cannabis use predicts more psychotic symptoms in people with schizophrenia. This is highlighted by the fact that the incidence of schizophrenia has decreased in recent years whilst the use of cannabis has increased. This may indicate that cannabis use is unlikely to have caused cases of schizophrenia that might not otherwise have occurred. Chronic cannabis use may precipitate schizophrenia in vulnerable individuals but not cause the underlying psychotic disorder, an effect that would not change reported incidence. Overall, those people with schizophrenia or a family history of schizophrenia are at a greater risk for adverse effects from the use of cannabis and are recommended strongly not to use it.
