The rate of marijuana-use disorders in the general U.S. adult population is 1.5%.4 Using resting state fMRI to compare connectivity of the brain reward circuit in 12 control subjects to that of 12 patients with co-occurring schizophrenia and cannabis-use disorder, the researchers found that the patients had significantly lower baseline connectivity in the brain reward circuit than controls. They also found that connectivity increased in patients after they were administered THC via smoking or capsule.

Another recent study, which appeared in the Journal of Psychiatry & Neuroscience, used event-related potentials to compare reward processing between 35 controls and 35 patients with schizophrenia spectrum disorders.1 The researchers further divided each group into cannabis users and nonusers, and assessed each participant’s response to pleasant images.

Their results demonstrate that, compared with cannabis-using control subjects, cannabis-using schizophrenia patients displayed a reduced response toward pleasant stimuli, and this blunted response predicted more frequent use of cannabis. “Our study suggests that reward processing deficits may promote cannabis use in individuals with psychotic illness,” said study co-author Clifford Cassidy, PhD, a post-doctoral research fellow at the New York State Psychiatric Institute at Columbia University Medical Center.

“Substance use could be an example of a behavior that could continue to be reinforced even in the presence of reward processing deficits since it has many advantages over more ecological rewards, being more immediate, more predictable, and more intense,” he told Psychiatry Advisor.

Regarding the debate about whether cannabis use causes psychiatric symptoms or patients use the drug to alleviate symptoms, Cassidy said, “I do not agree with this dichotomous way of thinking, and have greater interest in an alternate model, the common vulnerability model, in which both the psychiatric illness and the substance use could be secondary to some underlying pathology” such as the reward processing deficits observed in his study.

Findings of another study published last year suggest that a shared genetic etiology may be at least partially responsible for the connection between schizophrenia and cannabis use.5 For many patients, “it may be a less consciously-driven process whereby their brain reward systems are just tuned in such a way to be driven toward actions that lead to more immediate reward such as substance use,” said Cassidy.

Given the increased rates of cannabis use among people with schizophrenia, along with the role of the endocannabinoid system in the pathophysiology of the disease, some experts have begun investigating whether some cannabinoids could help manage some of the symptoms of schizophrenia.