Progressive gray matter volume reductions have been found in schizophrenia and greater changes seem to be related to poorer outcome1,2. As patients with schizophrenia who use cannabis have a worse prognosis 3, the progressive gray matter change in these patients might be even greater.

Fifty-one patients with recent-onset schizophrenia (cannabis users n=19; non-users n=32) and thirty-one matched healthy comparison subjects were included in this five year longitudinal MRI study. All subjects were assessed at inclusion and after five years. Total brain, gray and white matter, cerebellar, lateral and third ventricle volumes were measured. Percentages of volume change over time were calculated. Univariate analysis of covariance and pairwise comparisons were performed.

Cannabis using patients, non-using patients and healthy comparison subjects differed significantly in total brain, gray matter, lateral and third ventricles and cerebellum volumes. No change in white matter was observed between the groups.

Cannabis using patients with schizophrenia showed a more rapid decrease in total brain and cerebellar volume and increase in lateral and third ventricle volumes as compared to healthy subject and non-using patients. Gray matter volume decrease occurred in all patients with schizophrenia as compared to healthy subjects, but was significantly greater in patients using cannabis.

In schizophrenia progressive gray matter volume decrease occurs during the first five years of illness. Cannabis use causes an additional decrease of gray matter in patients with schizophrenia and could be explained by either a worse illness outcome or the effects of cannabis.