More recently, Hall and colleagues60 concluded that "there is reasonable evidence that heavy cannabis use, and perhaps acute use in sensitive individuals, can produce an acute psychosis in which confusion, amnesia, delusions, hallucinations, anxiety, agitation and hypomanic symptoms predominate." Regardless of which of those interpretations is correct, the two reports agree that there is little evidence that marijuana alone produces a psychosis that persists after the period of intoxication.
The association between marijuana and schizophrenia is not well understood. The scientific literature indicates general agreement that heavy marijuana use can precipitate schizophrenic episodes but not that marijuana use can cause the underlying psychotic disorder.59,96,151 As noted earlier, drug abuse is common among people with psychiatric disorders. Estimates of the prevalence of marijuana use among schizophrenics vary considerably but are in general agreement that it is at least as great as that among the general population.134 Schizophrenics prefer the effects of marijuana to those of alcohol and cocaine,35 which they seem to use less often than does the general population.134 The reasons for this are unknown, but it raises the possibility that schizophrenics might obtain some symptomatic relief from moderate marijuana use. But overall, compared with the general population, people with schizophrenia or with a family history of schizophrenia are likely to be at greater risk for adverse psychiatric effects from the use of cannabinoids.
As discussed earlier, acutely administered marijuana impairs cognition.60,66,112 Positron emission tomography (PET) imaging allows investigators to measure the acute effects of marijuana smoking on active brain function. Human volunteers who perform auditory attention tasks before and after smoking a marijuana cigarette show impaired performance while under the influence of marijuana; this is associated with substantial reduction in blood flow to the temporal lobe of the brain, an area that is sensitive to such tasks.116,117 Marijuana smoking increases blood flow in other brain regions, such as the frontal lobes and lateral cerebellum.101,155 Earlier studies purporting to show structural changes in the brains of heavy marijuana users22 have not been replicated with more sophisticated techniques.28,89
Nevertheless, recent studies14,122 have found subtle defects in cognitive tasks in heavy marijuana users after a brief period (19-24 hours) of marijuana abstinence. Longer term cognitive deficits in heavy marijuana