There is some clinical support for the hypothesis that heavy cannabis use may cause a psychosis that would not occur in the absence of cannabis use, the symptoms of which are preceded by heavy cannabis use and remit after abstinence. There is more support for the hypothesis that cannabis use may precipitate schizophrenia or exacerbate its symptoms. A large prospective study has shown a linear relationship between the frequency of cannabis use by age 18 and the risks over the subsequent 15 years of a diagnosis of schizophrenia. It is unclear whether this means that cannabis use precipitates schizophrenia, whether it is a form of "self-medication," or whether the association is due to the use of other drugs, such as amphetamines, which heavy cannabis users are more likely to use. There is stronger evidence that cannabis use can exacerbate the symptoms of schizophrenia. References