Cannabis use during adolescence is linked to an increased risk of depression and suicidal behaviour in young adulthood, a new study has found.

But the level of increased risk of depression and suicidal thoughts and attempts found in United States study is only low to moderate. No link was found to anxiety.

The research combines the results of 11 separate studies published over the past 15 years that together included more than 23,000 adolescent cannabis smokers and assessed their mental health when aged 18 to 32. People with prior depression were excluded.

"This review both confirms and reinforces findings from the research literature on the adverse psychological effects of regular cannabis use by mid- to late adolescents," said Dr Joe Boden, the deputy director of the University of Otago's Christchurch long-term health and development study.

"The findings of this [US] study further reinforce our concerns about the public health implications of any changes we may choose to make to cannabis laws in New Zealand," Boden told the Science Media Centre.

Giles Newton-Howes of the Medical Cannabis Research Collaboration says it's something that should help shape our referendum.

But he told Larry Williams it should also be put in perspective.

"What this tells us is that there are clear negative outcomes from cannabis over time. In saying that, there are some severe negative outcomes from alcohol, and anyone who doubts it should spend an evening in the ED."

He says that this is not the first piece of research to reach this conclusion, but this one stands out as it followed the same group of people over time. 

"I think for us in New Zealand, this research is important is because the people included in the research are like us, and some of them are us, coming from a New Zealand based study."

Newton-Howes says that the belief is that because teenagers brains are changing so quickly, that's why cannabis affects them so severely. 

"We know there are a lot of negative, short term consequences. The really important thing about this research is that those consequences don't seem to be time-limited, there seem to be negative consequences into adulthood."

He says that if we change how we regulate cannabis, there will be an increase in adolescent use, based on what has happened overseas.

"Making sure that we get the wording right is going to be really important for that referendum." 

The Government announced in December it will change the law to treat use of all illegal drugs as a health matter - and crack down harder on the suppliers of synthetic drugs.

Boden has written previously that there is growing evidence that regular or heavy use of cannabis may increase risks of: mental health problems, other forms of illicit drug use, dropping out of school and educational underachievement, and car crashes and injuries.

The new study, published in the Journal of the American Medical Association, does not state how much cannabis the research participants smoked, which is considered a significant omission.

And the study type could not show causal links. British experts have pointed out that as well as cannabis possibly affecting later mental health, it is plausible that people prone to mental health problems are more likely to smoke cannabis.

Dr Lindsey Hines, of the University of Bristol, said it was already known that using cannabis coincided with anxiety, depression and self-harm in teenagers.

While the US study suggested a link between early cannabis use and later issues, "we don't know if cannabis use as a teenager is causing these adult mental health problems.

"It could be that these behaviours are all due to shared underlying risk factors, such as early adversity or genetics."

Professor Sir Robin Murray, a psychiatry researcher at King's College London, said that although the modest risk increase found in the US study was probably real, better-quality studies had found cannabis use increased the risk of schizophrenia-like psychosis more than the risk of depression or anxiety.

He also noted limitations in the US study, including that the researchers had not specified the quantity or type of cannabis smoked.

"We know from studies of psychosis that the risk is much greater with daily use of modern, high-potency … cannabis than old-fashioned … varieties. We also don't know the extent to which the subjects were using other drugs or tobacco, which may be important.

WHERE TO GET HELP:

If you are worried about your or someone else's mental health, the best place to get help is your GP or local mental health provider. However, if you or someone elseis in danger or endangering others, call police immediately on 111.

OR IF YOU NEED TO TALK TO SOMEONE ELSE:

• LIFELINE: 0800 543 354 or 09 5222 999 within Auckland (available 24/7)
• SUICIDE CRISIS HELPLINE: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• YOUTHLINE: 0800 376 633 ,free text 234 or email [email protected]or online chat.
• NEED TO TALK? Free call or text 1737 (available 24/7)
• KIDSLINE: 0800 543 754 (available 24/7)
• WHATSUP: 0800 942 8787 (1pm to 11pm)
• DEPRESSION HELPLINE: 0800 111 757
• SAMARITANS – 0800 726 666.