Rationale: The relationship between cannabis and schizophrenia is complex. Some studies indicate that cannabis increases the risk of schizophrenia in those who are predisposed. On the other hand, cannabis is one of the illicit substances most commonly used/ misused by schizophrenia patients. Schizophrenia patients, or those vulnerable to psychosis, may attempt to self-medicate with cannabis.

Method: This study evaluated the effects of cannabis in 13 schizophrenic patients and compared these to control subjects. Subjects were tested on 3 separate days, following either 2.5 or 5.0 mg Delta-9-THC or placebo in a randomized, counterbalanced order under double-blind conditions. Symptoms of schizophrenia, along with cognitive, motor, and endocrine functions were measured.

Results: Some symptoms of schizophrenia got worse with THC exposure. THC also impaired the ability to remember new information and led to more abnormal movements in schizophrenics. In addition, THC increased levels of the stress hormone cortisol, and produced associated feelings of anxiety in schizophrenics.

Meaning: This well-controlled laboratory study suggests that cannabis is not an effective treatment for schizophrenia, but rather, worsens some symptoms. The high rates of cannabis use among schizophrenics, despite negative effects, may increase the burden of this debilitating disease.