An Edmonton-area drug bust that netted four kilograms of a synthetic opioid 100 times more toxic than fentanyl has prompted a warning to emergency room doctors and the public.

W-18, a synthetic opioid with no known clinical use, is considered 10,000 times more powerful than morphine and 100 times more potent than fentanyl — a similar narcotic responsible for about 270 overdose deaths in Alberta last year.

"Fentanyl has taken hundreds of lives in Alberta, and we believe W-18 is a more significant threat," Staff Sgt. Dave Knibbs said Wednesday at a news conference in Edmonton.

He talked about the powdered drug seized by Alberta Law Enforcement Response Teams (ALERT) during a bust in December 2015 that investigators didn't recognize. They sent it for testing, and Knibbs said they received official confirmation Tuesday that the drug was W-18.

It's the first time W-18 in powder form has been seized in Alberta.

'More overdoses and deaths'

Emergency room doctors in Edmonton received a warning about W-18 days before the public was notified, in a memo obtained by CBC News.

"ALERT seized four kilograms of W-18 powder as a result of an investigation into a clandestine lab in Edmonton," health officials wrote in an email to emergency department leaders, dated April 15.

"We may see increasing numbers of overdoses/deaths linked to W-18."

'Fentanyl has taken hundreds of lives in Alberta, and we believe W-18 is a more significant threat,' says Staff Sgt. Dave Knibbs. (CBC) Knibbs said four kilograms of powder would be enough to produce millions of tablets. Investigators intercepted the batch before it was processed and put on the streets.

W-18 is not yet a controlled substance in Canada. ​

On Tuesday, Alberta Justice Minister Kathleen Ganley and Health Minister Sarah Hoffman wrote a letter urging the federal government to speed up the process that would see W-18 added to the list of illegal drugs under the Controlled Drugs and Substances Act.

"It's a very dangerous drug," Ganley said in an interview. "People need to understand that this can be very deadly at a very, very small dose."

Ganley said the federal government has already given notice that it plans to add W-18 to the list of controlled substances.

"I think they can probably move quite quickly," Hoffman said, "and I hope that they will. I'm always very hopeful that when it comes to life and death situations that we're moving at breakneck speed."

The Alberta ministers also asked the federal government to add the precursors used to make fentanyl to the list of substance covered under the legislation.

Ultra-toxic W-18 traces its origins to medical research conducted in Edmonton more than 30 years ago.

W-18 was largely unheard of in Alberta until a drug bust last August put the law enforcement community on alert.

In what was the first publicized seizure of the drug on record in Canada, three of 110 pills seized from a home in Calgary tested positive for the narcotic.