Policies governing the use of cannabis need to avoid criminalising the large number of occasional recreational users who pose no risk to themselves or others, says a Christchurch medical researcher.

Professor David Fergusson, of the Christchurch School of Medicine, said although cannabis may have harmful effects on a minority of heavy or vulnerable users, there was little evidence to suggest the occasional use of cannabis was harmful.

"However, potentially large numbers of young people who use cannabis are, in theory at least, at risk of criminal prosecution for their actions.

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