Recent studies suggest that those who smoke cannabis are twice as likely to develop schizophrenia as nonsmokers. One widely publicized 2007 review of the research even concluded that trying marijuana just once was associated with a 40% increase in risk of schizophrenia.
Schizophrenia is a psychotic disorder (or a group of disorders) marked by severely impaired thinking, emotions, and behaviors. Schizophrenic patients are typically unable to filter sensory stimuli and may have enhanced perceptions of sounds, colors, and other features of their environment. Most schizophrenics, if untreated, gradually withdraw from interactions with other people, and lose their ability to take care of personal needs and grooming.
Case #1: One study led by Dr. Serge Sevy, an associate professor of psychiatry at the Albert Einstein College of Medicine in New York City, looked at 100 patients between the ages of 16 and 40 with schizophrenia, half of whom smoked marijuana. Sevy and colleagues found that among the marijuana users, 75% had begun smoking before the onset of schizophrenia and that their disease appeared about two years earlier than in those who did not use the drug. But when the researchers controlled for other factors known to influence schizophrenia risk, including gender, education and socioeconomic status, the association between disease onset and marijuana disappeared.
Case #2: In another study published in the British Journal of Psychiatry, researchers tracked the moods of 80 marijuana smokers, 42 of whom had psychotic disorders like schizophrenia. Participants were asked to record their moods at various points over the course of six days, determined by a watch that beeped periodically to signal the volunteers. All participants, not surprisingly, reported feeling happier when they were high, but the mood-lifting effect of marijuana was stronger among smokers with schizophrenia. Unlike people without the disease, schizophrenia patients also reported a reduction in negative feelings after smoking marijuana. "Everyone feels better," explains lead author Cecile Henquet, an assistant professor of psychology and psychiatry at Maastricht University in Holland. "But schizophrenia patients also have less anxiety and are less socially withdrawn."
Case #3: In a second study by Sevy at Albert Einstein College of Medicine, researchers interviewed adults with schizophrenia (and their families) who smoked marijuana, and found that they reported being better adjusted during childhood than those who did not indulge. It makes sense when you consider the practicalities and social nature of drug use. Being at least somewhat socially connected is necessary to be able to obtain an illegal drug - if you appear too "crazy," people are less likely to befriend you and dealers may be too wary to sell to you. Many people with schizophrenia exhibit odd behavior that puts them at risk for social rejection years before they develop full-blown delusions and hallucinations in adulthood. (source)