A new study into Kiwis' use of cannabis has found most of us have tried it by adulthood "with relative impunity, escaping serious health and or social consequences".
But a "small portion" experienced "non-trivial negative outcomes", the University of Otago researchers said, including "impairment in psychological function, loss of cognitive capacity, poorer respiratory and gum health, and a range of negative psychosocial consequences such as early school leaving and academic underachievement, failure in employment and the workplace, dependence on social benefits, and risk of criminal conviction or incarceration".
The researchers looked at data from two long-running studies in Dunedin and Christchurch to see how the drug has impacted Kiwis' lives, ahead of this year's referendum on whether recreational use should be legalised - aiming to provide an "agnostic" and "cool-headed" view of the evidence so people could make up their minds.
They found 80 percent of Kiwis born in the 1970s have used it at least once, despite it being illegal.
"The illegal status of marijuana does not prevent most people from using, and arrests and convictions do not lead to a reduction in use, and often see a bias against Māori," said director of The Dunedin Multidisciplinary Health and Development Study, Prof Richie Poulton.
About 5 to 10 percent of users were deemed at-risk, mainly because they used it more days than not, were dependent on the drug or had started early, developing a cannabis habit as teenagers they haven't been able to shake as adults.
"One thing is very clear from the research - and thankfully it appears to be a message that has been widely understood and accepted - regulations that restrict access to use by children and adolescents must be prioritised and enforced," the study said.
The data has been taken from longitudinal studies tracking the lives of a couple of thousand people born in Dunedin and Christchurch in the 1970s - often considered world-class in terms of the data they provide.
The researchers found about 15 percent of Kiwis had used the drug by the time they were 15, and between 70 and 80 percent by their 20s. Around half of them used cannabis at least once a year in their 20s, which would have been the 1990s/early 2000s.
Usage dropped as people aged - by 45, only a quarter had smoked cannabis in the previous year, with 6.1 percent being regular users and 2.1 percent dependent. In comparison, about 80 percent of Kiwi adults drink at least once a year, a quarter of them drinking hazardously according to Ministry of Health guidelines.
Heavy use of cannabis was associated with lower chances of employment and financial difficulties, as well as use of other illegal drugs.
Māori were more likely to be arrested or convicted for cannabis-related offences than non-Māori who had the same level of cannabis use. Arrests had no effect on reducing cannabis use.
"Nearly all of those arrested or convicted either continued using cannabis at a similar rate or increased their use, post-arrest," the study said. "That is, arresting or convicting people for cannabis use did not stop them from using."
Prof Poulton said the successful public health response to the COVID-19 pandemic - which has the country on the verge of wiping out known transmission of the deadly disease - shows the value of sound scientific advice.
He said he's been "giving evidence to various health select committees about cannabis use and harms for almost a quarter of a century" urging politicians to treat drugs as a health issue, not criminal.
"In an era where decisions are often skewed by scruffy information, ideological bent, special interest groups and commercial pressures, science is more important for society than ever. Our study takes away emotion and agenda, and lays the scientific facts about the impacts of cannabis on the table. This is the value of science."
The referendum will take place alongside this year's general election in September.