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New research reveals how long it takes for cannabis impairment to subside

Roadside drug testing does not account for impairment levels, like alcohol testing does.(ABC North Coast: Gemma Sapwell )

New research has shown for the first time how long cannabis users are likely to be impaired and when it may be safe for them to drive. Key points:

  • The research analysed 80 scientific studies on impairment from THC

  • Levels of impairment depended factors like dosage and how often a person used

  • The TGA has approved 100,000 prescriptions for medicinal cannabis in Australia

The findings, researchers and advocates say, strengthen the case for changes to drug-driving laws in much of Australia.

Researchers from the Lambert Initiative for Cannabinoid Therapeutics at the University of Sydney discovered users were impaired for between three and 10 hours after taking moderate to high doses of the intoxicating component of cannabis, tetrahydrocannabinol (THC).

THC can be detected in the body for weeks after cannabis consumption, meaning users can face fines and loss of their licence, despite being unaffected by the drug.

The research, published in Neuroscience & Behavioural Reviews, analysed 80 scientific studies on the effect of THC on driving performance conducted over the past 20 years.

Danielle McCartney hopes her study will help to inform cannabis users and policymakers.(Supplied) It found the exact level of impairment depended on the dose, whether the THC was taken orally or inhaled and how often the person used the drug, among other factors.

“Our analysis indicates that impairment may last up to 10 hours if high doses of THC are consumed orally," the study's lead researcher Danielle McCartney said.

"A more typical duration of impairment, however, is four hours, when lower doses of THC are consumed via smoking or vaporisation and simpler tasks are undertaken."

Dr McCartney said people could be impaired for six or seven hours if higher doses of THC were inhaled and complex tasks, like driving, were assessed.

Her research is the first comprehensive meta-analysis to put a timeframe on impairment.

"Our evidence should hopefully help people to make informed decisions and policymakers to make policies that are evidence-based and tell people how long they should wait before driving," she said.

The Therapeutic Goods Administration (TGA) has approved 100,000 prescriptions for medicinal cannabis in Australia.

A drug test used by police to detect impaired drivers.(Supplied: ACT Policing) Academic director of the Lambert Initiative, Iain McGregor, said medicinal cannabis users were particularly interested to know when it was safe for them to drive, despite the law being clear on the issue.

"You got this massive amount of a prescription drug going into people who are told, 'You can't drive at all, you can't even have one molecule of THC in your system', which is, you know, just ridiculous," Professor McGregor said.

"THC can be detected in the body weeks after cannabis consumption while it is clear that impairment lasts for a much shorter period of time. Our legal frameworks probably need to catch up with that."

Former magistrate David Heilpern said the research showed laws around roadside drug testing needed to change.

Former magistrate David Heilpern is calling for fairer drug-driving laws.(Supplied: Harm Reduction Australia) Mr Heilpern retired early party due to his frustration at seeing so many medicinal cannabis patients lose their licence, and sometimes their livelihoods, after being caught driving with small amounts of THC in their system.

"We had a situation where people were taking their medicine as prescribed, they weren’t driving in any adverse way and yet they were losing their licence, being fined and getting a criminal record,' he said.

"I started driving home from work, thinking, I just can't do this.

"I really found it difficult to live with myself."Mr Heilpern said cannabis was the only drug in Australia that you could get by prescription but could not drive with even a detectable level.

He is part of the Cannabis Law Reform Alliance, which is advocating for the amendment to state laws, providing medicinal cannabis users a defence if they test positive to a roadside drug test.

The defence already exists in Tasmania and there are bills before parliament in Victoria and South Australia. NSW parliament rejected a bill on the issue in October.

"In NSW, we already have that law as it applies to morphine, Mr Heilpern said.

"If you have a detectable level of morphine in your system and you can show you have a prescription for it, then you have a defence.

"All we have to do is do that for cannabis. It’s a very simple amendment and it solves the problem." There are calls for medicinal cannabis users to be allowed to drive.(ABC News)Gino Vambaca, co-founder of Harm Reduction Australia, said Australia's laws punished people for past drug use, not for unsafe driving.

"It's not a road safety campaign anymore, it's a detect and penalise campaign," he said.

"We're not condoning people using drugs and driving, but what we’re saying is, there's no attempt by the police to even measure impairment.

"We’re having to say to people using medicinal cannabis: ‘Do you want to drive or do you want your pain relief, because you can't do both.’

"And that’s a horrible choice for them to have to make."


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