More Evidence Cannabis Use Linked to Schizophrenia

Cannabis use causes a temporary cognitive breakdown in nonpsychotic individuals, leading to long-term psychosis, new research shows.

In an imaging study, investigators at the University of Bergin, in Norway, report that they found a different brain activity pattern in schizophrenia patients who had a history of cannabis use compared with schizophrenia patients who had never used cannabis.

"While brain activity for both groups was similar, there are subtle differences between schizophrenia sufferers with a history of cannabis use and those who have never used cannabis. These differences lead us to believe that the cognitive weakness leading to schizophrenia is imitated by the effects of cannabis in otherwise nonpsychotic people," lead author Else-Marie Løberg, PhD, University of Bergen, said in a statement.

The study was published in the October issue of Frontiers in Psychiatry.

Cannabis and Cognition

"We know that the short-term effect of cannabis is negative on neurocognition and that cannabis use is also a risk factor for psychosis," Dr. Løberg told Medscape Medical News.

"We also know that the typical patient with nonaffective psychosis has clinically significant cognitive deficits that are stable and chronic, even after their psychosis has been treated.

"So we are proposing that cannabis mimics these deficits, turning the individual to psychosis."

The investigators carried out a functional magnetic resonance imaging study during which they measured brain activation during alternating cognitive task-present and task-absent conditions in 26 schizophrenia patients with and without a history of cannabis use.

Patients were scanned while they were asked to listen to different syllables in each ear and to say which syllable was spoken when instructed to concentrate on either the left or the right ear.

The researchers note that this is a difficult task for anyone but that it is particularly difficult for patients with schizophrenia, who often have impaired attention, limited executive functioning, and difficulty in processing verbal cues.

Growing Evidence

Researchers found that patients who had previously used cannabis had consistently higher levels of brain activity while undergoing this test as well as a higher number of correct answers than patients who had not previously used cannabis.

In contrast, patients who had not previously used cannabis had higher levels of brain activation in the absence of the task compared with prior users of cannabis.

"We now have international data that support the fact that cannabis is a risk factor for psychosis for some, though not all, patients with schizophrenia, and those patients with a positive cannabis history need help to understand the negative effect of cannabis on psychosis," Dr. Løberg said.

"At the same time, these patients may have fewer cognitive deficits and thus may have a better chance of optimal community outcome, if abstinent, as opposed to many nonaffective psychosis patients."

Eden Evins, MD, MPH, Harvard Medical School, Boston, Massachusetts, told Medscape Medical News that the study does not in fact show that cannabis changes cognitive performance.

"It's been shown numerous times that among those with and without schizophrenia, certain cognitive skills are required to obtain illicit drugs, so there may have been differences in brain function prior to cannabis use which they didn't test for."

Dr. Evins also noted that patients with no history of cannabis use had more normal activation patterns during rest, which is actually a good thing in that this mode has been shown to be important for learning in healthy individuals.

The authors and Dr. Evins have disclosed no relevant financial relationships.

Front Psychiatry. Published online October 30, 2012. Abstract