Scientists and lawyers are raising a series of concerns over Ottawa's plans to combat drug-impaired driving, saying the proposed regime is not based on evidence and will struggle to withstand legal challenges.

Bill C-46, which would create new drug-impaired driving offences, is currently being studied in the Senate, where there is growing pressure on senators to amend the proposed legislation before it comes into law. The government wants the new rules in place before cannabis is legalized for recreational use, a move expected in late summer.

According to experts, Bill C-46 sets limits for levels of THC, the primary psychoactive compound in cannabis, that do not determine whether a driver is impaired or not. Cannabis acts differently on the body from alcohol, and there is no exact link between driving abilities and THC levels, as measured in nanograms per millimetre (ng) of blood.

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The "per se" limits for alcohol (80 milligrams of alcohol in 100 millilitres of blood) have been upheld in court, but the limits for cannabis have not been tested in court. Some frequent users have constant levels of THC and are not actively impaired, while others may be impaired with low levels of THC.

"As toxicologists, we're not big on 'per se' numbers for drugs because drugs are very different from alcohol on how they impair people, the tolerance and the wide spectrum of tolerance that exists between different individuals," D'Arcy Smith, a member of the RCMP's drug evaluation and classification program, told senators last month.

Amy Peaire, who chairs the drugs and driving committee at the Canadian Society of Forensic Science, added impairment depends on a variety of factors, including whether cannabis was smoked or eaten. Cannabis is fat soluble, which means blood concentration levels vary between individuals.

"Unlike alcohol in which you can have blood concentrations which have links to impairment, that's not the same for THC because there's not a good correlation between impairment and blood concentration," said Ms. Peaire.

Justice Minister Jody Wilson-Raybould has defended her government's approach, stating it relied on scientific evidence, including the work of Mr. Smith and Ms. Peaire.

"We're moving forward on the drug levels that we have in C-46. The levels have been set and based in large part on the advice provided to us by the drugs and driving committee. The science around drug levels is something that continues to evolve and we'll continue to engage with the committee," Ms. Wilson-Raybould told reporters.

Under Bill C-46, three new offences would be created:

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Having a THC level between 2 and 5 ng, punishable by a fine of up to $1,000;

Having a level above 5 ng, punishable by a fine of $1,000 for a first offence and escalating penalties for repeat offenders;

Having a blood alcohol concentration of 50 milligrams per 100 mL of blood in addition to a THC level superior to 2.5 ng, with the same penalties as the previous offence.

Ms. Wilson-Raybould said the best way to prevent drug-impaired driving is simply to stay off the road after consuming cannabis.

"Until we have definitive scientific evidence around what is a safe level, we're proceeding on a precautionary basis, saying there is no safe level," Ms. Wilson-Raybould said.

But lawyers warned the government the new regime will be hard to enforce, with legal challenges guaranteed to clog up Canadian courts already beset by delays.

"The [Canadian Bar Association] does not view Bill C-46 as offering any significant improvement over the existing legislation, certainly not one that would offset its negative impact on justice efficiencies," spokeswoman Kathryn Pentz told senators.

Ms. Pentz added the new law is bound to be challenged in court and that prosecutors will be hard-pressed to defend the measures.

"It has to be evidence-based. At this point and time, the experts are saying we really cannot get a correlation between a level of drugs in one's system and a level of impairment," she said.

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Conservative Senator Claude Carignan has expressed serious reservations about Bill C-46, saying the best way to determine whether drivers are impaired is through certified drug recognition experts (DRE). However, the process to become a DRE is long and mostly requires training in the United States, leaving Canada with a shortage with only a few months to go before legalization.

"The solution will be to present an amendment to Bill C-46 to suspend the application of the 'per se' limits up until the moment that scientific evidence backs up their use, and to rely solely on DREs in the meantime," Mr. Carignan said.

Kyla Lee, a Vancouver-based defence lawyer, said the law would be more likely to withstand a constitutional challenge if DREs were used in every case before charges are laid.

"I believe it would be more likely to withstand Charter scrutiny," she told senators.