No evidence? Not quite. The latest research on the risks of cannabis

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In November, Victoria’s voters put cannabis legalisation firmly on the political agenda. Legalise Cannabis won two seats in the state’s upper house and parties favourable to legalisation picked up another six seats. To govern, the re-elected Labor government will likely need their support - meaning the government may need to play ball, as my colleague Jewel Topsfield writes.

Legalise Cannabis Victoria’s upper house victories sparked jubilation after the state election.Credit: Jason South

There’s local and global momentum for the legalisation of cannabis. The Age has editorialised in favour of a serious discussion about reform.

That’s the politics. What about the evidence?

When Professor Wayne Hall, former director of the National Drug and Alcohol Research Centre, began researching cannabis, “any evidence of harm was taken at face value”, he tells me. “Now, we’ve had this flip. There’s a super-scepticism of any evidence of harm.”

Indeed, Legalise Cannabis claims there is “no convincing evidence that even heavy, long-term use by adults permanently impairs” cognitive function.

I’m not convinced the evidence supports that. The risks of cannabis use are low – far lower than many legal drugs, such as alcohol – but the evidence suggests they are not zero. If we’re going to legalise this drug, we should all be clear-eyed about that – even supporters.

What do we know about the risks?

First, cannabis is safer than many other drugs in important ways. It does not come with an overdose risk. It does make you a more dangerous driver, but that risk is lower than for people who drive when drunk.

Cannabis comes with a risk of dependence, which varies – depending on what study you look at – between 1 per cent and 40 per cent for people who have used it daily since their teenage years (Hall suggests a range of 20 to 40 per cent is more accurate as the true risk of dependence if you have ever used cannabis).

Cannabis smoke contains similar carcinogens to tobacco vapours. But linking it to lung cancer has been difficult, probably because there are few pack-a-day cannabis smokers.

A mature marijuana plant on an unidentified legal farm in NSW.Credit: Janie Barrett

It is with mental health and cognitive function that the evidence is most interesting. Cannabis derives its potency from compounds that closely mimic key signalling molecules within our endocannabinoid system, a complex of sensors enmeshed in our brain and nerves.

The highest quality evidence of harm we have suggests an increased risk of psychotic or schizophrenia symptoms. Heavy users of cannabis are almost four times more likely to have schizophrenia and psychotic symptoms compared to non-users. However, the absolute risks here are very small: you’d have to stop 4700 young men smoking cannabis to prevent one case of schizophrenia. (Interestingly, the vast majority of cannabis use is by men at every age range.)

Without randomised controlled trials it has been hard to establish cause, rather than association. Perhaps people with psychotic symptoms are more likely to be heavy users of cannabis as a form of self-medication.

But we now have a cool study that can help us solve this. Researchers in London ran a large study looking at the relationship between psychosis and the strength of cannabis a person was using, and found that higher-strength users were more likely to report psychotic disorders. While people with psychosis might self-medicate with cannabis, it seems unlikely they would prefer high over low strength - so what we’re really seeing is the effects of the cannabis.

What about brain function? A systematic review suggests long-term use affects verbal learning and memory. Attention and processing speed are also affected, even in cannabis users asked to abstain for 23 days. It remains unclear if brain function returns to baseline levels after a long period of abstinence, or if the changes are permanent.

The best study we have, Hall says, comes out of Dunedin, New Zealand, where scientists have been following 1037 people since birth.

It found that people using cannabis at age 15 had more schizophrenia symptoms at 26, even after controlling for any symptoms they may have shown as children. A follow-up study at age 38, using the same group, reported that the heaviest users (those who started early and used cannabis four days a week or more) lost an average of five or six IQ points from age 13 to 38.

What happens when we legalise it?

It’s hard to know what effect legalisation will have. The US, where 21 states have legalised recreational use, is still in the first throes of a grand experiment. But early evidence suggests legalisation leads to a steep drop in the price and a sharp increase in the potency of the drug; a whole pharmacopoeia of cannabis products, some at extremely high concentrations, are now available. Research suggests legalisation increases use, as you’d expect.

If the harms of cannabis are not zero, increasing its use will likely lead to an increase in harm. Indeed, after its legalisation in Colorado, hospitals reported higher rates of patients visiting emergency with cannabis and mental health complaints.

But that does not necessarily mean we should ban the drug. We allow people to use substances that prove far more harmful with regular use, such as alcohol, tobacco and opioids. And what if cannabis use replaced rather than supplemented alcohol use? That would likely be a public health boon.

We may be able to legislate to avoid the worst harms: high taxes could discourage heavy daily use, and education programs could target young people, as we know they face the biggest health risks. But that’s easier said than done. Australia has an enviable record on tobacco control, but has been less successful in other areas such as gambling.

Hall says: “If you look at the US or Canada … we’ve just had this massive expansion in hyper-potent cannabis products, with a lot of promotion. My worry is we’ll see something like the gambling industry, that becomes too profitable to regulate – and that’s exactly what we’re seeing in the US.”

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