About 75 per cent of New Zealanders have used cannabis at least once by the age of 25, according to The Christchurch Health and Development Study.

"That suggests trying cannabis is a normal part of growing up in New Zealand," University of Otago associate professor Dr Joseph Boden said.

Legalised cannabis would unlikely increase that figure any further. This country's "high rates of cannabis use" suggest we are already at saturation point.

"If we're already at 75 per cent of young people who have tried it, then there isn't many more young people who are going to come on board.

​"The ones who want to are already doing it."

Boden said the majority of cannabis use among young people was experimental or occasional; "there are not a lot of people using it heavily".

While it was difficult to predict if the number of users would increase under legalisation, he thought "it would be small".

University of Queensland Centre for Youth Substance Abuse professor Wayne Hall said legalising the drug would likely have the most significant impact on current users.

Hall said if cannabis was made more affordable and easier to access then consumption would increase, like any commodity.

"The people who already enjoy using it when it's illegal will use more heavily and more frequently when it's cheaper and decriminalised."

Those who used cannabis frequently (more than once a week) were most likely to suffer effects to their health and wellbeing.

The commonly held perception that cannabis was a harmless drug was wrong, said Massey University drug researcher Dr Chris Wilkins.

"If you're using high potency, using daily or if you've a history of mental illness or drug addiction it can have serious health problems bringing on mental illness or further addiction. Continue reading

• Laura Baker is a life and style reporter at Fairfax Media

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For the past one year, their self-determined spiritual quest has been to heal illnesses with the help of marijuana.

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