courage a person to use marijuana heavily, such research can only be conducted on people who become users on their own. An epidemiological survey could be used to identify such young people and might shed light on the relationship between motivation and marijuana use. But while such a study might show that marijuana users tend to lose motivation compared with nonusers, it could not be used to establish that marijuana use causes people to become unmotivated.

A major question remains as to whether marijuana can produce severe and lasting psychotic disorders. There are clinical reports of marijuana-induced states that resemble psychoses such as schizophrenia, depression, and mania, with symptoms that last a week or more. Some researchers have argued that the diversity of these symptoms belies the existence of a specific “marijuana psychosis.” Others have concluded that heavy marijuana use—and perhaps even acute use in especially sensitive people—can produce a psychosis characterized by a suite of symptoms such as confusion, amnesia, delusions, hallucinations, anxiety, and agitation. Regardless of which of these interpretations is correct, both camps agree that marijuana use alone—without the influence of additional risk factors—is unlikely to provoke a psychosis that persists longer than intoxication.30

Drug abuse is common among people with mental illness. Thus, it is not surprising that several (but not all) studies have shown that a disproportionately large number of people with schizophrenia use marijuana. The association between marijuana and schizophrenia is not well understood, however. While experts generally agree that heavy marijuana use can provoke schizophrenic episodes in susceptible individuals, they also concur that the drug does not cause the underlying disorder. Additional research indicates that people with schizophrenia prefer the effects of marijuana over those produced by alcohol and cocaine, which they generally use less often than does the general population. The reasons for this preference remain unknown, but it suggests that marijuana might give these patients some relief from their symptoms. But people with schizophrenia or a family history of the disease should understand that using marijuana puts them at a greater than average risk for adverse psychiatric reactions.31

Some of marijuana's psychological effects may prove to be

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