The Royal Australasian College of Surgeons has called for the repeal of Canberra's cannabis laws, saying they will lead to more death and injury on the road.
The group's call came as Health Minister Greg Hunt released a dossier of research showing marijuana use increases the risk of psychosis and mental health problems.
Mr Hunt also upped pressure on ACT Chief Minister Andrew Barr, with a spokesman saying Mr Hunt would be writing directly to Mr Barr to ask for the medical evidence he has that differs from the international literature.
"Legalising recreational cannabis is dangerous and medically irresponsible," he said. "The brief confirms the very real risks cannabis can pose to physical health and in particular, to mental health."
The federal government is collecting ammunition that could support an intervention to overturn the ACT's law before it takes effect at the end of January. Such an intervention would not be made lightly, given it would be a step-change to overturn territory laws outside issues of conscience such as euthanasia.
Trauma chairman of the surgeons group John Crozier said the ACT should consider the voices of "the people who pick up the pieces", and heed the calls of the police commissioners and deputy Prime Minister and minister responsible for road safety Michael McCormack, who has labelled the laws crazy.
Dr Crozier said surgeons witnessed the carnage from road crashes every day and were "duty-bound to speak with concern".
"The college has been absolutely consistent over many many years, chilled by the evidence of the coroners' courts and with the knowledge of what we deal with within the walls of our hospitals every day," he said.
"We know as day follows night, people who drive at under the influence of cannabinoids are at risk of death and serious injury to themselves, and critically to other road users."
The surgeons group did not make a submission when the ACT Assembly held an inquiry before passing the law in September. GPs group the Australian Medical Association, did make a submission, opposing the legalisation of cannabis, citing a possible five times increased risk of schizophrenia.
What a dopey decision the ACT Government has made in legalising cannabis.— Michael McCormack (@M_McCormackMP) October 3, 2019
As the Federal Minister with overall responsibility for road safety, I cannot for the life of me understand the logic behind this.
Read my @dailytelegraph op-ed👇https://t.co/qXfguHT3y7
Dr Crozier cited a 2013 paper which concluded the risk of a car accident doubled in the first one to two hours after cannabis smoking. While drivers drove more slowly after smoking cannabis, they also had less control when the task got more complex, with more lane weaving and slower reaction times, it found.
He cited a study reported in 2001 by the Victorian Institute of Forensic Medicine, which looked at 3400 fatalities in Victoria, NSW and Western Australia and found that drivers with cannabis detectable in their blood had a three-fold higher risk of being killed in a crash. Recent cannabis use was similar to driving with a blood alcohol concentration of 0.1 (twice the legal limit).
"This is a reliable set of data from Australia, this is real-world stuff," he said. "Recent use of cannabis significantly degrades cognitive skills and motor skills and objectively increases crash risk; this is particularly more likely in somebody using it for the first time."
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The Victorian researchers pointed to the flaws in roadside drug testing, finding no increased risk of accident among drivers who still test positive to cannabis from previous use, but no longer have it in their blood. Impairment could only be measured by a blood test, they said. Dr Crozier also raised concerned about roadside drug tests, saying they tested a metabolic product that could linger many days after use, but not the psychoactive chemical that impaired driving.
Dr Crozier also cited two US studies. One from the US Insurance Institute for Highway Safety found an overall 6 per cent increase in crash claims after cannabis was legalised in Colorado, Washington and Oregon.
But the other does not support the suggestion that cannabis increases crashes. The large-scale study from the US National Highway Traffic Safety Administration said the evidence was well-established that drugs could seriously impair driving ability. But it said an apparent 1.25 times higher risk of road crash among people testing positive to THC was due to other factors such as age, sex and ethnicity - an finding it said was in line with other studies.
Also on Monday, Mr Hunt released a briefing paper on the health impacts of marijuana. It said research in Colorado after cannabis was legalised showed daily or near-daily use was associated with the development of a psychotic disorder.
A 2014 study in Australia showed regular cannabis users had double the risk of psychiatric symptoms including schizophrenia. A review published in Canada in 2018 also showed an association with mental health problems.
The THC content of cannabis had increased by almost 30 per cent over the past 20 years, an increase thought to have worsened anxiety, depression and psychosis in new uses, and psychosis and dependence in regular users, the paper said.
Studies suggested a 1.5 times higher risk of developing depression, and brain changes and cognitive problems.
Mr Hunt's briefing paper on health impacts had the caveat that no study had shown a causal link, instead using the description "attributable". And it said a minority group of researchers argued that the mental health disorders might pre-date the individual's cannabis use.
The ACT has legalised cannabis, to take effect from January 30. Adults aged 18 or over are allowed two plants; households are allowed four. It will remain illegal to buy or sell cannabis. The ACT has not changed the laws on drug-driving, so it will remain illegal to drive with any detectable cannabis in your system. Some US states that have legalised cannabis have also introduced legal driving limits.
Despite the ACT parliament's decision, the legal status of cannabis remains unclear because it is still illegal to possess cannabis under federal law. Federal Attorney-General Christian Porter is consulting with the federal police and has said publicly he expects police to enforce Commonwealth law.
The government is considering its next step and has the power to overturn the ACT's law because of Canberra's status as a territory rather than a state.
Mr Barr has accused the Coalition of becoming "obsessed with small-scale personal use within the ACT" .