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The influence of drug use in the development of sz is controversial. While drug use may be associated with more rapid precipitation, an earlier onset age, and with poorer functioning and outcome, such as precipitating relapse, its role as an independent risk factor is unclear.
The main type of substance abuse, is alcohol abuse, either solely or in combination with other substances. Perhaps more research has been conducted on other drugs because of their psycho-trophic effects. Eg abuse of dopamine-releasing drugs - such as amphetamines and cocaine - can precipitate psychosis; cannabis appears to have similar risk-increasing effects, though over a longer period of time.  Indeed individuals with psychosis use cannabis more often than other individuals in the general population. It has thought that cannabis may be used self-medicate psychotic symptoms.
Recent epidemiological studies give (1) limited support for the hypothesis that cannabis use causes a psychotic disorder that would not have occurred in the absence of cannabis use; (2) recent prospective studies give stronger support that cannabis use may precipitate schizophrenia or exacerbate its symptoms; these relationships are stronger in people with a history of psychotic symptoms and they have persisted after adjustment for potentially confounding variables; and (3) there is also reasonable evidence that cannabis use exacerbates psychotic symptoms.
Diagnostic specificity is weak, as cannabis exposure may be a risk factor for many psychiatric disorders, ranging from schizophrenia to mood and anxiety disorders.
There may be a sex difference in drug use: alcohol and marijuana use is more common in young men. However whether this contributes to the higher rate of mental illness among young men is unclear. Furthermore, there may be a familial risk – drug interaction where individuals with a genetic loading for schizophrenia may be more likely to develop psychosis following cannabis abuse. Also individuals who inherit a schizotypal personality may be more likely to take drugs to try to alter an unhappy internal mood state.
There is biological plausibility to the idea that drug use may be a risk factor because some drugs (such as hallucinogens, amphetamine, and alcoholic withdrawal) are known to cause transient psychotic-like reactions, while other drugs (such as cannabis and amphetamine) can produce paranoia in non-psychotic individuals. Thus alcohol and drug abuse are not specific to schizophrenia, but may be examples of a ‘secondary’ hit which precipitate or aggravate the course of the disorder.
Again, there is the question of whether individuals with certain genotypes may selectively expose themselves to drugs to whose psychotogenic effects they are particularly vulnerable. Eg a large Taiwanese study found that methamphetamine abusers who developed a psychosis were distinguished from those who did not by a greater frequency of schizoid and schizotypal traits in childhood and by having more relatives affected with schizophrenia.