When Max Hill looks you in the eye, he still has the piercing gaze of the drug cop he used to be.

Key points: Veteran David Hill has PTSD and was being treated with benzodiazepines, a minor tranquiliser, at a psychiatric ward

Veteran David Hill has PTSD and was being treated with benzodiazepines, a minor tranquiliser, at a psychiatric ward When he was discharged he deteriorated further as he withdrew from the medication, but medicinal cannabis helped

When he was discharged he deteriorated further as he withdrew from the medication, but medicinal cannabis helped When the medicinal product was no longer available, his father Max turned to the black market. The pair now want to help others

Now in retirement, he's still hunting cannabis suppliers. But this time, it's as a customer on behalf of his son, David — an army veteran of two tours of Afghanistan who uses it to treat post-traumatic stress disorder (PTSD).

"I'm a bit slow on the uptake, I suppose," Max said.

"It took me 22 years in the police force, and 20 years since, to learn that cannabis is a medicine."

This is no 'Breaking Bad' story. Max is openly and defiantly flouting a law he says has passed its use-by date.

His turning point came when David — who was suffering terribly with PTSD — left a psychiatric ward after being treated with massive doses of benzodiazepines.

David said the medication made some symptoms worse.

"The benzodiazepines were not a positive impact on my life whatsoever," David said.

"They initially took away the pain. But then it caused addiction, then overt destructive behaviour. It caused me to cut off from other people. It almost exacerbated my symptoms."

Alone in his apartment, David was deteriorating fast as he tried to come off his medication.

"He was withdrawing terribly," Max said.

"Totally dishevelled, crying uncontrollably… incapable of managing anything other than lying in bed and looking at his phone."

David himself doesn't have much memory of that time, describing himself as a "medicated zombie".

David did two tours of Afghanistan during his time serving with the Australian Army. ( Supplied: David Hill )

Cannabis helped ease brutal withdrawal

Benzodiazepines, also known as minor tranquillisers, are most commonly prescribed by doctors to relieve stress and anxiety and to help people sleep.

They can also be used to treat alcohol withdrawal and epilepsy.

The Australian Alcohol and Drug Foundation says there is increasing concern among medical professionals about the risks of benzodiazepines, particularly when used for a long time. Withdrawal can be brutal.

And so it was for David Hill.

A month after discharge from hospital, Max brought his son back to the family home to care for him.

David had also been prescribed medicinal cannabis to ease his withdrawal.

At first, it seemed to have little effect.

"I think we came very, very close to losing him," Max said.

"For six long weeks I gave him every meal, every drink, every medication."

Then — quite suddenly — David's condition rapidly improved.

"He's woken up one day, and we're talking like a light switch. He was back to, as I put it, 93 per cent human again. At his depths, he was about 10 per cent human — if that," Max said.

What happened next convinced Max it was cannabis making the difference.

The product they'd been prescribed was no longer available, and the substitute — a much weaker version — wasn't having anything like the same effect.

He'd also seen research from the US and Israel about how cannabis had been used to treat PTSD there.

David believes other veterans with PTSD could benefit from access to subsidised medicinal cannabis. ( Supplied: David Hill )

Turning to the black market

But then, there was the cost — nearly $800 a week.

David pleaded with the Department of Veterans Affairs to help pay for it, but got nowhere.

That's when Max — the former drug cop who used to bust bikies selling dope — decided it was time to go to the black market himself.

"I was beyond caring, frankly," he said.

"What do you do? DVA wouldn't pay for it, despite repeated requests. They wouldn't even give reasons in writing as to why they wouldn't pay for it."

The Department of Veterans Affairs spends $230 million a year on mental health support for veterans.

It has not closed the door on medicinal cannabis, but says right now there is only "limited evidence" supporting the use of the drug for mental health conditions including PTSD.

"DVA funds treatment where there is a clear evidence base," a spokeswoman said.

"However, the research into medicinal cannabis as a treatment is rapidly evolving and DVA will continue to monitor the developing research and published evidence."

The department does have a framework for funding medicinal cannabis for some veterans with chronic pain, chemotherapy-induced nausea and vomiting, and multiple sclerosis.

In the meantime, David continues to use black-market cannabis to manage his PTSD symptoms.

"My anxiety and depression has dwindled down to nearly nothing," he said.

"I'm sleeping a full night's sleep, waking up fully refreshed with motivation, with a clean mind. It's really nice."

David is telling his story in the hope it might help other veterans who are struggling with PTSD. ( ABC News: Sean Warren )

Push to legalise, and subsidise, cannabis

David Hill's experience is no scientific study.

But it tallies with results from an open trial of CBD — the therapeutic element in cannabis — on severe anxiety in 30 young people conducted in Melbourne's west.

Professor Pat McGorry says the trial at Orygen Youth Health found "encouraging results along the lines that David experienced".

"We now have major funding to test it as a preventive treatment in young people who are in the early stages of psychosis," Professor McGorry said.

David says he's telling his story on Anzac Day because he's lost too many mates to suicide.

He fears for those going through the same treatment he did.

"Talking to them, you notice they're sluggish, unmotivated… it's a walking, unconscious being that you're talking to," David said.

"That's where I was before this treatment my father found for me. And I just want them to be able to reap the benefits that I have, and see them put their lives back together.

"There are laws being broken, but they're outdated laws.

"I'd like to see it available, I'd like to see it legalised, I'd like to see it subsidised by the department, and I'd like to see it pushed — because I am a case study. It has worked 100 per cent."

If you or someone you know needs help you can contact Open Arms, a free and confidential counselling service for Australian Defence Force members, veterans and their families, on 1800 011 046.