Reports from various researchers have suggested that marijuana-induced
psychosis is generally short-lived and that total remission can be expected.
Such reports, however, have been based on case studies, not on long-term
follow-up data, according to the authors of a new, long-term study.
The study found that an episode of marijuana-induced psychosis is not
innocuous—it often presages subsequent psychotic episodes and a
diagnosis of a schizophrenia-spectrum disorder.
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.
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."
An abstract of "Cannabis-Induced Psychosis and Subsequent
Schizophrenia-Spectrum Disorders: Follow-Up Study of 535 Incident Cases"
is posted at<http://bjp.rcpsych.org/cgi/content/abstract/187/6/510>.▪
Br J Psychiatry
2005187510