The Canadian Safety Council has problems with legislation before Parliament designed to crack down on drug-impaired drivers.
"I think it is driven by hype. I think it is driven by politics," the council's Emile Therrien told Â鶹´«Ã½ on Monday.
The bill introduced by the Conservative government gives police powers to take blood or urine samples if they suspect someone behind the wheel is high.
The council estimates that 500,000 Canadians in any given year toke and drive.
But there is no foolproof test for drug impairment in the way that a breathalyzer can detect if someone is over the legal limit.
"It is very difficult to establish the scientific basis or the technology to establish impairment levels for these drugs," Therrien said.
Defence lawyers say that testing for the mere presence of a drug could lead to wrongful convictions.
"What may show up in their urine or their blood will be remnants of THC that is evidence the person smoked marijuana -- when? That day? Two days (ago)? Three days?" said defence lawyer Lawrence Greenspon.
He doesn't think the legislation would withstand a constitutional challenge.
Mothers Against Drunk Driving defend the bill and say it is necessary.
"It will help reduce the number of deaths and injuries on the highways and Canadians will be safer," said MADD president Karen Dunham.
Justice Minister Rob Nicholson is confident the technological challenges can be overcome.
"We've had a look at the science and we are confident that this will work," he said.
Opposition MPs are not convinced.
"We should perhaps not be saying they have the right to demand whether it is a urine or a blood sample," said Joe Comartin, the NDP's justice critic.
"Presumably there are empirical ways to prove people are drug impaired. We have yet to see it," added Liberal MP Brian Murphy.
Robert Fife, CTV's Ottawa bureau chief, said opposition will likely support the bill despite their reservations. No party wants to be accused of being soft on impaired driving.
The council said the best approach to combat drug driving is to get the provinces to invoke administrative license suspensions as they do with drivers who have imbibed alcohol but are not over the legal limit.
If police suspect someone is under the influence of drugs they could take their license for 24 hours.
With a report from CTV's Robert Fife
