Marijuana is often called a gateway drug, but a new study suggests it could actually have the opposite effect for those who already use stronger substances such as heroin and methamphetamines.

Dr. M-J Milloy conducted a study that suggests hard drug users are 55 per cent less likely to report an overdose if they also use cannabis products.

"We don't know the specific reason why, but if could be, of course, related to the fact that we know that there is not fentanyl in cannabis, but fentanyl has turned up in crystal methamphetamine, it's turned up in cocaine, it's turned up opioids—really the entire list of drug supply is contaminated by fentanyl," he told CTV.

According to Milloy, the evidence also suggests that that cannabis consumption helps prevent drug users from transitioning to injection heroin, which is the deadliest method of consumption and increases a user's risk of contracting diseases such as HIV in the process.

"This seems to indicate that cannabis might be almost sort of an ad hoc harm reduction strategy by people who are trying to change their use of other drugs," Milloy said.

He's planning a clinical trial which will be made much easier when recreational use of cannabis becomes legal in October.

Meanwhile, some of those who are helping make cannabis available to hard drug users say they've experienced the benefits first-hand.

Melanie Pratt was 39 years old when a heroin needle broke off in her arm and quickly became infected.

She was rushed to hospital after she came to a day and a half later. A surgeon managed to save the arm, but gave her a dire warning.

"I could continue using with one arm and I could probably die doing it," she said. "That was May 1, 2006. That was the last time I ever used. For me, it was a no-brainer. They were going to take my arm. There was no going back for me."

After three months at St. Paul's Hospital, Pratt found marijuana helped her cope with the symptoms of withdrawal as she fought to kick the habit.

"It just kept me peaceful, it kept me functional," she said. "I could eat, I could sleep, I could function and I don't abuse it, and I could put it down and be OK with it."

It's been 12 year since then, and Pratt now works for High Hopes, a program that offers cannabis products to those dealing with additions to harder drugs.

"It helps a lot of guys that are coming in here and they're actually staying off heroin," she said. "It just makes you feel better… If you're not withdrawing or feeling any pain, then you feel good, right? And I just think it's a lot less harmful than other drugs."

Pratt said marijuana can help with everything from pain, stress, and loss of appetite to sleep deprivation, anxiety and depression.

High Hopes sells its products in an open-air market in Vancouver's Downtown Eastside.

The pot is sold at cost, but those who don't have any money can get help for free.

"Anybody that wants to help themselves, especially to get off something as dangerous as heroin, I'll help them," Pratt said.

The program is run by the Overdose Prevention Society. OPS director Sarah Blyth said of the 100 people currently in the program, 25 have quit heroin and as many as 50 have reduced their heroin consumption.

"It gives them an alternative to drugs on the street," she said. "It's safe, it can reduce pain, it can reduce trauma."

And there's good reason to avoid street drugs in Vancouver: much of the current supply is contaminated with fentanyl, a highly-toxic synthetic opioid that is at least partly to blame for the 878 lives lost to overdoses in B.C. so far this year.

Now, there's some science to back Blyth and Pratt's on-the-ground of experience of marijuana's potentially life-saving benefits to drug users.

"They use cannabis as a safe alternative to opiates and alcohol and other things and it's really working out to be something really interesting," Blyth said.

With files from CTV Vancouver's Jon Woodward