The potential for cannabis-related harm is being downplayed in debates about the drug's legal status, according to Otago researchers.

An editorial in the New Zealand Medical Journal by research Associate Professor Joe Boden and the late Emeritus Professor David Fergusson urges caution before altering laws, citing evidence from two of Otago's long-running population studies.

"Most contributions [to the debate] imply that cannabis is a relatively harmless drug, and that cannabis law change will only have beneficial consequences," they wrote.

"We would argue that, on the basis of evidence generated by longitudinal studies based in New Zealand, both assumptions are incorrect."

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The Christchurch Health and Development Study and the Dunedin Multidisciplinary Health and Development Study provide some of the most comprehensive data on cannabis-related harm in the world.

Cannabis use by those involved in the Christchurch study was associated with educational delay, welfare dependence, increased risks of psychotic symptoms, major depression, increased risks of motor vehicle accidents, tobacco use and illicit drug use, and respiratory impairment, Newshub reported.

Their proposal discouraged the use of cannabis, but proposed cautious decriminalisation and harsher penalties for supplying cannabis to those under 18.

Emeritus Professor Fergusson, the former director of the Christchurch Health and Development Study, dedicated almost 40 years of his life to the research until he died in October last year.

Meanwhile another study released last week claimed to show that using cannabis even once or twice as a teenager had the potential to affect the grey matter in the brain.

Led by Dr Catherine Orr, of Melbourne's Swinburne University of Technology, the study found a link between that effect in some sites and increased anxiety.