Investigators at the University of Berlin assessed the impact of cannabis on cognitive functions in schizophrenic patients who reported prior use of pot versus patients who reported no history of substance abuse. Researchers reported that cannabis use was not associated with any decline in cognition, and that those subjects who reported using marijuana prior to their first psychotic episode showed improved cognitive performance on certain tests compared to non-users.

"[T]o our surprise, cannabis abusing schizophrenic patients … achieved results either similar to those [achieved] by the non-using cannabis schizophrenic patients or, at times, performed even better than them," investigators concluded. "[R]ather than deteriorating neuropsychological performance, cannabis [use] prior to [a patient’s] first psychotic episode improved cognition in some tests."

According to the study’s authors, cognitive dysfunctions are present in more than 80 percent of patients diagnosed with schizophrenia.

A separate 2005 study by investigators at Manchester Metropolitan University in Britain previously reported that schizophrenic patients who consumed cannabis prior to disease onset possessed greater cognitive skills after ten years than did non-users.

Neurocognitive studies performed on healthy volunteers generally report that the use of marijuana, even long-term, is not associated with any significant or long-lasting declines in cognitive function.