Schizophrenia, a chronic mental illness, is often accompanied by poorer cognitive functioning, and some research has suggested that marijuana may worsen schizophrenia. But this case-control study shows enhanced cognitive functioning in schizophrenic subjects who use marijuana. Schizophrenic study participants with lifetime marijuana use demonstrated better performance than those without use on a component of psychomotor speed. Frequent or recent users tended to show better neuropsychological performance, particularly in attention/processing speed and executive functions.

C.M. Coulston, M. Perdices and C.C. Tennant, “The neuropsychological correlates of cannabis use in schizophrenia: lifetime abuse/dependence, frequency of use, and recency of use,” Schizophrenia Research 96, issue 1-3 (November 2007): 169-184.
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