Cannabis users have higher premorbid IQ than other patients with first onset psychosis

A number of studies have reported that patients with psychosis who use cannabis have better cognitive performance than those who do not. This is surprising as cannabis can impair cognition in healthy subjects. An obvious question is whether the better current performance of psychotic patients who have used cannabis is a reflection of their having a higher premorbid IQ than those psychotic patients who haven't used cannabis.

In a sample of patients at their first episode of psychosis, we tested the hypothesis that patients who smoked cannabis would have a higher premorbid IQ than patients who did not.

279 participants (119 patients and 160 healthy controls) were assessed in order to obtain current and premorbid IQ measures and detailed information on cannabis use. We examined the association between cannabis use and both premorbid and current IQ in patients and controls.

Patients who had ever smoked cannabis had significantly higher current (p<.001) and premorbid IQ (p=.004) compared to patients who had never used cannabis. This difference was not found among controls.

These findings suggest that the better cognitive performance of patients with their first episode of psychosis who have used cannabis compared with those who haven't is due to the better premorbid IQ of the former.