Mental Illness, such as major depression and bipolar disorder, can be treated effectively with cannabis in many cases.  Some researchers have attempted to link mental illness to marijuana use, but there is no clinical evidence that cannabis use directly precipitates mental illness in general, or any specific mental disorder. However, it is reasonable to assume cannabis use might catalyze latent mental illnesses in some cases. Patients suffering from severe schizophrenia, should not use any psychoactive medications without a physician’s approval. However, research from pharmacologist Daniele Piomelli in 1999 does not appear to corroborate anecdotal evidence indicating that cannabinoids might help stabilize schizophrenia.
More recent research found improved neuropsychological performance following cannabis use. According to the author of a 2007 study from Berlin, "[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."
Cognitive dysfunctions are thought to be present in more than 80 percent of patients diagnosed with schizophrenia.
A separate 2005 study by investigators at Manchester Metropolitan University in Britain 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. 
Related sections: Depression, Psychoactivity, Stress Reduction.
 Grinspoon and Wilson, “Marijuana and Bi-polar Disorder.” The Forbidden Medicine Website, http://www.rxmarijuana.com
 Hollister, “Health aspects of marijuana.” Pharmacological Review, Vol. 38, No. 1, 1986