There is growing evidence that cannabis use can contribute to schizophrenia. Some studies suggest that cannabis is neither a sufficient nor necessary factor in developing schizophrenia, but that cannabis may significantly increase the risk of developing schizophrenia and may be, among other things, a significant causal factor. Nevertheless, some previous research in this area has been criticised as it has often not been clear whether cannabis use is a cause or effect of schizophrenia. To address this issue, a review of prospective cohort studies has suggested that cannabis statistically doubles the risk of developing schizophrenia on the individual level, and may, if a causal relationship is assumed, be responsible for up to 8% of cases in the population.