Mikkel Arendt, Ph.D., a fellow at the Center for Basic Psychiatric Research at the University of Aarhus in Denmark, and coworkers used the Danish Psychiatric Central Register to identify patients treated for a first marijuana-induced psychotic episode between 1994 and 1999. There were 535 such patients. The researchers then followed those patients for at least three years to determine how many of them experienced subsequent psychotic episodes and how many could be diagnosed with a schizophrenia-spectrum disorder.

The researchers found that 77 percent of the subjects incurred subsequent psychotic episodes and that 45 percent could be diagnosed at some time within the next three years or more with a schizophrenia-spectrum disorder. Moreover, of the 45 percent who developed a schizophrenia-spectrum disorder after experiencing marijuana-induced psychosis, 37 percent received such a diagnosis within three years and the remaining eight after three years. Furthermore, those who developed such a disorder did so at an earlier age than did comparison subjects—individuals who developed such a disorder but who had no recorded history of marijuana-induced psychotic symptoms. This effect was most marked for paranoid schizophrenia.

“An episode of short-lived psychotic symptoms following cannabis use seems to have great prognostic value.”

Thus, “for the majority of patients, cannabis-induced psychotic symptoms proved to be a first step in the development of a schizophrenia-spectrum disorder or other severe psychopathology,” Arendt and his group concluded in their study report, which was published in the December 2005 British Journal of Psychiatry. The results do not prove that marijuana is causally linked with schizophrenia, the researchers stated; owing to the study design, it was not possible to control for potentially confounding factors such as hereditary predisposition, socioeconomic status, or other kinds of drug use. Nonetheless, the researchers concluded, marijuana use might well hasten the onset of schizophrenia since the subjects who developed schizophrenia in the wake of using marijuana did so at a younger age than the comparison subjects.

“I think it is important to follow the patients treated for cannabis-induced psychosis closely and to offer them and their relatives information on risk factors for, and early signs of, schizophrenia,” Arendt told Psychiatric News. “Much work is going on around the world trying to find early signs of schizophrenia because the prognosis of patients improves with early intervention. An episode of short-lived psychotic symptoms following cannabis use seems to have great prognostic value.”

Eric Strain, M.D., a professor of psychiatry at Johns Hopkins University and chair of APA’s Council on Addiction Psychiatry, agreed. “The study suggests that a substance-induced psychotic episode serves as an important indicator identifying a group of patients at high risk for subsequent psychiatric needs.”

Nonetheless, Arendt stressed, “Our study does not show that marijuana is a risk factor for young people with a family history of schizophrenia. However, the next phase of our investigation will be a study on hereditary predispositions among those developing schizophrenia following an episode of cannabis-induced psychosis.”

Joan Arehart-Treichel