Scientists at the University of York have shown that the risk of developing psychosis, such as hallucinations, from cannabis use is small compared to the number of total users. The research, published in the journal, Addiction, also showed for the first time that there is sufficient evidence to demonstrate that for patients who already have schizophrenia, cannabis makes their symptoms worse. In order to prevent just one case of psychosis, more than 20,000 people would have to stop using cannabis, as shown by a previous study led by the University of Bristol.