A university professor says there are a series of negative mental effects of cannabis. Voters will be asked to answer yes or no at next year's election to whether cannabis should be legalised. Associate Professor of Psychiatry at Otago University Giles Newton-Howes told Mike Yardley research has shown the impacts of the drug are more pronounced the younger a person uses it. "We can be pretty confident to tell young people using cannabis increases their risk of psychosis. Some of the most severe form of that being illnesses like schizophrenia." He says if the drug is legalised, it's likely it will be more widely consumed. "There's a very broad range of both data, commentary and opinion that make it really clear that if you increase availability, you increase use. If you increase the acceptability in terms of social context, you increase use."