On Monday, the federal Liberals approved a road side saliva testing device for marijuana impairment, and it's already drawing criticism.

The Drager DrugTest 5000 and the training to use it will now be available as an option for police to test impairment from THC — the main psychoactive ingredient in cannabis — ahead of legalization in October.

Kyla Lee, a Vancouver criminal defence lawyer who specializes in impaired driving cases, says that questions around the accuracy and invasiveness of the test will mean many court challenges.

"The biggest impact we're going to see right off the bat, is that it's going to open the door to a lot of constitutional challenges," Lee said.

Lee says that the time the test takes to conduct makes it problematic. She says that the saliva collection takes one to four minutes, plus 10 minutes for the device to read to the cartridge containing the saliva, plus another 10 minutes of buffer time before the saliva is even taken to ensure no food or drink skews the test results.

"The problem is that roadside impairment testing is supposed to be done immediately which is not going to happen with this device," she said.

"It's really out of step with what the courts have authorized insofar as roadside testing for impaired driving."

Furthermore, Lee says that the collection of a saliva sample containing someone's DNA is very different than offering up a sample of breath as one would for a breathalyzer.

"Saliva is very different. We're not all going around giving the police or government samples of our saliva."

And even once the saliva is collected, there is no definitive number as to what constitutes impairment from THC, she says.

"Unlike alcohol, where you can smell liquor on somebody's breath or see evidence of consumption in other ways, cannabis is different," she said.

"There are going to be numerous court challenges across the country to this."

And because the whole process is so new, Lee says judges will likely have conflicting rulings from province to province.

"The Supreme Court of Canada is going to have to sort it out, which means millions of dollars and several years before we have an answer about whether or not this is actually something that should be taking place on our roadways."