The study, published July 9, 2014 in the journal Schizophrenia Bulletin, followed 110 individuals with a schizophrenia diagnosis over an 18 month period to determine whether cannabis use had an effect on their illness.

"There was no evidence of a specific association between cannabis use and positive symptoms, or negative symptoms, relapse or hospital admissions," wrote Christine Barrowclough, PhD, a professor of clinical psychology at the University of Manchester and the study's lead author.

Symptoms in schizophrenia are split into two categories. Positive symptoms include hallucinations, delusions and unusual thoughts whereas negative symptoms include lack of pleasure, emotion and involvement in everyday life.

Scientists have long known that schizophrenics have higher rates of cannabis use than the general population. However, studies have provided conflicting evidence on whether marijuana use leads to better or worse outcomes in the long-run.

According to Dr. Barrowclough, the study was "methodologically robust" and one of the largest to investigate the subject to date.

Participants were recruited from five different treatment centres in England. The majority of patients were male, in their mid-20s and unemployed. Substance use was assessed at the start of the study and at 4.5, 9 and 18 months following. On average, participants used marijuana 4-5 days per week and consumed 1.3 grams per usage.

While cannabis use did not appear to have any direct impact on symptoms or outcomes in the current study, higher doses of cannabis were modestly associated with anxiety and poorer patient functioning. On the other hand, patients that used marijuana showed better functioning overall.

"Cannabis use was associated with an improvement in general functioning, a finding that was also evident in our earlier study with a sample of patients with longer illness history."

The contrast may indicate that the "impacts of cannabis on people with psychosis are quite complex and variable," offers the team. Interestingly, there is also some evidence that compounds in marijuana could be useful for treating the disorder.

The researchers conclude that further studies should be conducted to "take forward this clinically important area of study."

Source: LeafScience