The association of marijuana and schizophrenia (a life-long mental disease characterized by chronic hallucinations and delusions) may be one the most studied and most controversial areas in drug abuse (Hall W, Lancet 374, p 1383-1391, 2009). The most compelling data come from studies that found those who used marijuana were more likely to later develop schizophrenia than those who did not. Plus, several of the studies showed the more marijuana smoked the greater the risk. But showing something predicts disease later does not necessarily mean it caused it. The most common interpretation of all these studies is that marijuana use by itself cannot cause schizophrenia but can cause schizophrenia to occur earlier, be more severe when it occurs, and make treatment more difficult. In certain persons, marijuana also can cause an acute psychosis; i.e. having hallucinations and delusions that last for several days after marijuana use but, unlike schizophrenia, these soon go away. So when someone starts “acting crazy” after using marijuana, often doctors have to simply wait and see if it will go away or stay on as schizophrenia.
–Dr. John R. Hughes