When the pain gets too bad, Julie* puts a couple of drops of cannabis oil under her tongue, to ease her chronic pain and help her sleep.

Key points: Driving with any THC in your blood system is a criminal offence in Australia

Driving with any THC in your blood system is a criminal offence in Australia Standard medical advice for medical marijuana patients is to wait for five days after using it before driving

Standard medical advice for medical marijuana patients is to wait for five days after using it before driving Professor Iain McGregor says it is important to use science to look at the impairment THC in the bloodstream causes

It contains a low dose of THC — the psychoactive ingredient in cannabis.

Certainly not enough to get her anywhere near stoned, she says.

"Oh God, no. No way. I can barely tell the difference," she said.

"I don't like feeling stoned, I don't like smoking pot. It's definitely not about getting high."

And when she wakes up and drives to work, she knows she is breaking the law.

In Victoria, as in all Australian states, driving with any THC in your system is a criminal offence.

"I would never take it just before I hop into a car," Julie says.

"I just think if I get busted, I get busted, and I'll argue my case in court because I think the laws are stupid."

Driving with any THC in your blood system is a criminal offence in Australia. ( ABC News: Stephen Stockwell )

But cannabis patients who ignore the standard medical advice to wait five days before driving are risking more than a fine or a suspended licence.

If they are involved in an accident, their insurance will be voided.

If another person is injured, they could face criminal charges of driving while impaired.

Medical marijuana reduces patients' independence

Karen Hitchcock is a pain specialist who prescribes cannabis and says it is an extremely difficult decision for her patients to make.

"By the time they wake up in the morning, they won't be impaired at all, just as you wouldn't be if you had a few glasses of wine the night before, but they are still unable to drive for five days," Dr Hitchcock said.

Pain specialist Dr Karen Hitchcock prescribes cannabis to her patients. ( ABC News: Peter Drought )

"Often, people choose not to take the medicine because it results in such a huge decrease in their independence.

"I've had patients who have come to me on high doses of opioids and Valium, and they gradually use cannabis to wean off their other medications.

Law Institute of Victoria president Stuart Webb says other countries have provided some provision for people to drive with a level of THC in the blood system. ( ABC News: Chris Le Page )

"But because they can't tolerate the prohibition on driving, they say, 'I'm just going to go back on my opiates and Valium'."

The growth in medical cannabis has been astonishing.

At the beginning of last year, there were around five applications per month approved for medical cannabis products.

By December, it had rocketed up to 141.

That has bumped up against the toughest roadside drug testing regime in the world, which is also expanding.

Law Institute of Victoria president Stuart Webb says it is slightly anomalous.

"Other countries have looked at this and have provided some provision for people to drive with a level of THC with the blood system," he said.

"I think that we need to look at that."

Reason Party MP Fiona Patten says cannabis is being treated differently under the law to other prescriptions drugs like opioids and benzodiazepines.

Reason Party MP Fiona Patten says cannabis is the only prescription medication that completely excludes you from driving. ( ABC News: Chris Le Page )

"Cannabis is the only prescription medication that excludes you 100 per cent from driving," she said.

"There's an easy fix to this: include medicinal cannabis patients in our Road Safety Act to say that someone who is found to have a presence of THC is not breaking the law, unless they are driving when impaired."

Patient gives up licence to use THC oil

Until then, other patients, like Josh Gatehouse, decide the trade-off is worth it.

Mr Gatehouse, who has autism, gave up driving when he started taking a low THC oil to bring is racing thoughts under control.

It has enabled him to settle into study for the first time in his life.

"I just feel like I'm finally on a path somewhere," he said.

"I've being drifting for so long."

But in a small country town, giving up driving is a huge decision.

Josh Gatehouse and his mother Margaret say medical marijuana has been a real game changer. ( ABC News: Sean Warren )

The daily trip to TAFE used to be a 20-minute drive. Now, he has to take two buses each way.

"Yeah, it's inconvenient," he said.

"But I've never had this focus for a course — or anything before — in my life. And there are too many pros to outweigh the disadvantage of not having a licence."

His mother Margaret is close to tears when she talks about the change in her son.

"It's been a real game-changer for him. It really has," she said.

"I think it's a bit crazy, really, that they're not letting people drive. His medication has a very low level of THC.

"People are allowed to have a small amount of alcohol without it impairing them. I imagine that having a small amount of THC would be a similar thing. Perhaps even less of an impairment."

Impairment is at the heart of the debate

Professor Iain McGregor, from the Lambert Initiative into Cannabinoid Therapeutics at the University of NSW, says it is important to look at the complex relationship between THC levels in the human body and driving performance. ( ABC News: Billy Cooper )

"Do we want every patient who's taking medicinal cannabis to never drive? I mean, that's not feasible," said Professor Iain McGregor, from the Lambert Initiative into Cannabinoid Therapeutics at the University of New South Wales.

"Rather, we have to use science to look at impairment and the complex relationship between THC levels in your body and driving performance, rather than just having a very simplistic view that if you have THC in your saliva, you should be off the road."

But leaving that decision in the hands of drivers is a move cautioned against by Michael Fitzharris, associate director of Monash University's Accident Research Centre.

"They actually have that artificial feeling of wellbeing," Associate Professor Fitzharris says.

"While the skills that they need — like concentration and reaction time, when you need to respond — aren't always there."

"And that starts to present as a road safety problem, which is why it remains illegal to drive with any cannabis on board."

Michael Fitzharris, the associate director of the Monash University Accident Research Centre, says THC in the blood stream does present a road safety problem. ( ABC News: Peter Drought )

Mr Gatehouse has his own idea for a solution — re-testing medical cannabis patients while they are on their medication.

"I could go back and do the same tests that I did in the first place to check whether I'm capable of driving and reassess my ability as a patient," he said.

"It's like alcohol prohibition ending at the moment. Everybody's scrambling to see how it all fits together in the system."

*Name has been changed to protect the person's identity.