Records from more than 22,000 patients with psychotic illness reveals a link between marijuana use and early symptoms of psychosis.

Psychotic illness occurs significantly earlier among marijuana users, results of a meta-analysis suggest.
Information on more than 22,000 patients with psychosis showed an onset of symptoms about three years earlier among users of cannabis compared with patients who had no history of substance use.

"The results of this study provide strong evidence that reducing cannabis use could delay or even prevent some cases of psychosis," Dr. Matthew Large, of the University of New South Wales in Sydney, Australia, and co-authors wrote in conclusion.
"Reducing the use of cannabis could be one of the few ways of altering the outcome of the illness because earlier onset of schizophrenia is associated with a worse prognosis and because other factors associated with age at onset, such as family history and sex, cannot be changed."

Psychosis has a strong association with substance use. Patients of mental health facilities have a high prevalence of substance use, which also is more common in patients with schizophrenia compared with the general population, the authors wrote.
Several birth cohort and population studies have suggested a potentially causal association between cannabis use and psychosis, and cannabis use has been linked to earlier onset of schizophrenia. However, researchers in the field remain divided over the issue of a causal association, the authors continued.
Attempts to confirm an earlier onset of psychosis among cannabis users have been complicated by individual studies' variation in methods used to examine the association. The authors sought to resolve some of the uncertainty by means of meta-analysis.
A systematic search of multiple electronic databases yielded 443 potentially relevant publications. The authors whittled the list down to 83 that met their inclusion criteria: All the studies reported age at onset of psychosis among substance users and nonusers.
The studies comprised 8,167 substance-using patients and 14,352 patients who had no history of substance use. Although the studies had a wide range of definitions of substance use, the use was considered "clinically significant" in all 83 studies. None of the studies included tobacco in the definition of substance use.

The studies included a total of 131 patient samples.
Substance use included alcohol in 22 samples, cannabis in 41, and was simply defined as "substance use" in 68 samples.
Alcohol use was not significantly associated with earlier age at onset of psychosis.
On average, substance users were about 2 years younger than nonusers were. The effect of substance use on age at onset was greater in women than in men, but not significantly so. Heavy use was associated with earlier age at onset compared with light use and former use, but also not significantly different, the authors reported.
Substance users were two years younger at the onset of psychosis compared with nonusers. Age at onset was 2.7 years earlier among cannabis users compared with nonusers.
Acknowledging limitations of the study, the authors cited the lack of information on tobacco use and its association with earlier age at onset of psychosis, and the lack of data on individual patients inherent in all meta-analyses.
Despite the limitations, the authors said the findings have potentially major clinical and policy implications.
"This finding is an important breakthrough in our understanding of the relationship between cannabis use and psychosis," they wrote in conclusion. "It raises the question of whether those substance users would still have gone on to develop psychosis a few years later."
"The results of this study confirm the need for a renewed public health warning about the potential for cannabis use to bring on psychotic illness," they added.