Patients with bipolar I disorder performed better in neurocognitive assessments when they had a history of marijuana use. But that must be considered in the context of the known negative effects of marijuana in bipolar sufferers.

PROBLEM: At least one prior study has shown than cannabis might have some positive effects for patients with bipolar disorder, and several others have reported that in patients with schizophrenia, marijuana use is actually associated with an improvement in neurocognitive functioning. While it is still unclear why the psychoactive drug might have this effect on patients with major psychiatric disorders, this study further investigates the association in bipolar disorder patients.

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METHODOLOGY: Data was collected over nine years from patients at a Long Island hospital as part of a larger cohort study. All of the patients used for this study fit the DSM-IV requirements for bipolar I disorder, meaning they had experienced a true manic episode. Fifty individuals with a history of cannabis "abuse or dependence" and 150 individuals without this history took part in a number of standardized tests designed to measure their cognitive functioning. Researchers attempted to control for as many variables as possible, including their demographics, the age of onset and duration of the patients' illness and their estimated IQ from before they developed the disorder.

RESULTS: There was a general pattern of superior cognitive functioning in the group with a history of cannabis abuse. These patients performed better than their drug-free cohort on all significant measures, such as processing speed, attention, and working memory. The researchers suspected that this discrepancy could have been caused by increased alcohol dependence in the control group, but upon secondary analysis they found that alcohol abuse was higher in the cannabis group as well.

CONCLUSION: Patients with bipolar disorder demonstrated significantly higher neurocognitive performance when they also had a history of cannabis dependence. With these results added to what we know about schizophrenia, it is possible that the correlation is causal -- that marijuana use improves the cognitive functioning of patients with severe psychiatric disorders.

LIMITATIONS: The authors point out that a certain baseline of cognitive function is required for patients to get their hands on illegal drugs in the first place, which may provide another explanation for the correlation seen here. They also found that the patients in their sample with a history of cannabis dependence also had more severe cases of bipolar disorder, as manifested by an increased rate of psychosis during their manic episodes.

IMPLICATIONS: Marijuana has been more definitively shown to have negative effects on users with bipolar disorder -- making them less likely to comply with treatment or respond well to lithium (a common mood stabilizer), and more likely to have psychotic symptoms or attempt suicide. The authors of this study therefore suggest that the best next step might be to develop treatments that mimic the positive effects of cannabis without causing more harm than good.

The full study, "Cognitive and clinical outcomes associated with cannabis use in patients with bipolar I disorder," is published in the journal Psychiatry Research.