Police are getting access to a new tool to help them test whether drivers are high, less than two months before the legalization of recreational marijuana.

Despite serious concerns raised about the devices, Justice Minister Jody Wilson-Raybould approved the use of the Dräger Drug Test 5000.

The federal government will be providing $81 million to provinces and territories earmarked for the purchase of screening devices and training.

The tester allows officers to check drivers' saliva samples for the presence of THC – the ingredient in marijuana that provides the high. The device can also test for signs of amphetamines, opiates, cocaine, methadone and other drugs.

Lawyers are already raising red flags.

"I have concerns about the fact that police have never had any on-hand training with them or any experience with them," criminal defence lawyer Sarah Leamon said Tuesday.

"I also have concerns about whether or not they will be suitable for our Canadian environment."

The device doesn't work in temperatures below 4 C, and anyone who's had a drink or snack within 10 minutes may have skewed results.

It's prone to false negative and false positive results, critics say.

Officials will also have to decide what level of THC would be considered impaired, consider how long THC stays in the system, and if the results would stand up in court. Scientists can't agree on how much pot it takes to make someone too high to drive.

"With alcohol we know if you got .08 you're more impaired than if you got .05," explained University of Ottawa drug policy professor Eugene Oscapella.

"The results of drug tests don't really necessarily indicate impairment, all they can do is indicate fairly recent consumption of the drug, and that information has to be called in together with other information to try to indicate that you were actually impaired at the time you were stopped."

Positive results may give police the power to do more testing, such as a blood sample.

Lawyer Michael Spratt called the system a "recipe for constitutional disaster."

Describing the test system as a tool police can use to justify more invasive techniques, he said the machine's results can give officers the grounds to conduct an invasive search or handcuff a driver and take them back to a police station.

"What we know is studies show the machine is going to get that wrong 15 per cent of the time," he said.

Spratt said he doesn't think the results of the saliva test will be used in court, but that it still can represent a constitutional intrusion.

"We have to also remember that failure to comply with the police's demand to submit to this type of test – a test that is unreliable, that gets it wrong either way – is an offence in and of itself and can lead to criminal charges."

Another criminal defence lawyer, Mickelson & Whysall's Cathryn Waker, said marijuana can be detected in urine tests conducted on an occasional user for up to four days, and as long as 30 days in a chronic user.

And she said lawyers are gearing up to challenge detentions associated with testing.

"If you're detained at the roadside for 10 minutes, half an hour without access to counsel, that's definitely something that we'd be challenging in the courts," Waker said.

"You only can be detained at the roadside for a certain amount of time that is found to be a justifiable infringement of your rights… If there's found to be a violation of Section 9 (arbitrary detention) rights then whatever evidence is found after that gets found out."

While the approval drew criticism from some, it was welcome news to MADD Canada. A spokesperson said the government has carefully studied the lessons learned from countries where the Drug Test 5000 is already in use.

"We're using some of their research and case studies to help us frame the law," Gregg Thomson said.

"The truth of the matter is, as of Oct. 17 now, it will be legal in this country. We need something to protect the public."

Thomson lost his son in the 1999 in a crash involving a driver impaired by drugs. He said the test system is the last step in helping police enforce laws in place to save lives.

In a statement, the Department of Justice said use of the device was approved after a 30-day period allowing for public feedback.

A recommendation was also submitted by the Drugs and Driving Committee of the Canadian Society of Forensic Science. The group continues to evaluate other saliva-testing systems that could be used by law enforcement.

"The percentage of Canadian drivers who are fatally injured in vehicle crashes and test positive for drugs already exceeds the percentage who test positive for alcohol," Public Safety Minister Ralph Goodale said in the statement.

"The problem exists right now and we are implementing new tools to deal with it."

Goodale said officers are already trained to recognize signs of impairment and the testing system is an additional tool.

With files from CTV Vancouver's St. John Alexander