As New Zealand weighs up the pros and cons of cannabis legalisation ahead of our referendum, a study from the US has sounded a note of caution regarding the effect that that legalisation had on teenage cannabis use in one US state.
The longitudinal study, from the University of Washington and published in American Journal of Preventive Medicine, studied more than 230 teens and young adults in Washington state and found that teens may be more likely to use cannabis following legalisation than they otherwise would have been.
Recreational use of cannabis has been legal in Washington since December 2012 and the state was the first in the US to make the change.
Researchers looked at whether legalisation led to teens using cannabis and how they perceived harm caused by the drug.
The study found that teen use had been trending downward, with 11 per cent of young people born before 2000 reported using cannabis over the past year at age 15, but only 5 per cent born after 2000 reporting using the drug at the same age.
But crucially, young people interviewed at age 15 after legalisation were several times more likely to report use in the past year.
"When we think about marijuana legalisation, a worry is that underage use may go up," said Jennifer Bailey, the study's lead author.
"Early use and heavy use during adolescence can have a lot of negative health consequences, then and later in life, so we don't want teen use to be going up," she added.
The results differ from other studies, which showed that rates of teen use of cannabis dropped or held steady after legalisation.
The Washington study's authors say this may because their study was a better view of long-term trends, following young people born between 1989 and 2002 for 15 years.
The study also found that teens' perception of the harms caused by cannabis did not change after legalisation, with the study's authors noting that usage in recent decades is lower overall than it was in the 1960s and 1970s, when then was less information on the effects of cannabis and lower awareness or risk.
The study's authors concluded that legalisation "may be working against hard-won, population-level decreases" in cannabis use and recommended that jurisdictions looking to legalise cannabis "should devote increased resources to evidence-based prevention programming" for underage cannabis use.
What does the Cannabis Legislation and Control Bill do?
It would make it legal to use or grow cannabis for recreational purposes in New Zealand.
The production, supply and use of cannabis would be regulated by a new government-controlled authority.
Only people 20 years and older would be able to buy cannabis, and they would be able to buy up to 14 grams of dry leaves a day. That is also the maximum amount you are allowed to have in your pocket in public. It is enough to make up to 40 joints, and at black market prices would cost around $200.
You would not be able to light up a joint on the street, in a bar, or in your car. Smoking and consumption would be limited to your home or to specialised bars. "We don't expect a Amsterdam-style coffee shop culture," said Ross Bell, executive director of the NZ Drug Foundation. "Is it more likely to be a lounge room attached to a retail store."
The proposals for the cannabis industry are designed to keep it small, tightly regulated and out of sight.
You would be only be able to buy cannabis in licensed, physical stores. Online and remote sales would be banned, as would importing cannabis. There would be a total ban on marketing, advertising and promoting cannabis products, even inside of cannabis shops.
Cannabis potency would be restricted and clearly stated on a product's label - like the alcohol level on a beer bottle. Products would have to be sold in plain packaging and have health warnings - similar to cigarette packs. Edible cannabis products would also be available, but would be more strictly controlled.
The finer detail is yet to be worked out, but commercial supply would be capped at existing levels of demand, and reduced over time.
Companies would be limited to one part of the supply chain. For example, growers could not also be retailers. This is part of a plan to avoid a "Big Cannabis" takeover, as has been the case overseas. Tax on cannabis sales - which would be higher for more potent marijuana - would be channelled into harm reduction.
Home-growing would also be allowed. You would be able to grow a maximum of two cannabis plants at your house or rental - or up to four if there was more than one 20-year-old living at the property. There would be fines for growing too much, and potential jail time if you grew more than 10 plants. You could make edibles at home, but not resin, which can be more potent.
The proposed law change is fundamentally different from decriminalisation, in which cannabis possession and use remains illegal but is not punished with criminal charges. Medical cannabis is already legal in New Zealand.