A legal opioid now infiltrating North America's drug scene is being called the deadliest trend in more than three decades.

Police in Edmonton rushed this week to warn the public and emergency departments about W-18 after they seized four kilograms of the drug in powder form.

"To put in perspective, this is enough to kill every man, woman and child in Alberta about 45 times over," tweeted Edmonton public health doctor Hakique Virani about the amount of powder found in the bust.

Jim Hall has studied substance abuse for more than three decades. He said clandestine labs, mainly based in China, produce these synthetic products and ship them overseas using worldwide delivery services such as FedEx.

"This is the most deadly drug trend I've seen in 31 years," said Hall, an epidemiologist at Nova Southeastern University in Florida.

W-18 was invented by researchers at the University of Alberta in the 1980s, who developed a whole W series of drugs, from W-1 to W-32, for their pain-relieving potential. And although they were patented in 1984, the drugs had no known use outside of scientific research.

No W-18 antidote An April memo to Edmonton ER doctors explained there is "little evidence" that a well-known fentanyl overdose antidote, called naloxone, could be effective on W-compounds. Even so, naloxone is the only antidote recommended for suspected W-18 overdose patients. Alberta government officials say they're looking into ordering more naloxone kits for first responders.

Number 18 was the most potent — and therefore the most dangerous — of that series.

It produces a high like heroin, and Health Canada said it first appeared on the recreational drug scene in 2013 in Europe. Police first spotted it in Canada at the end of 2015.

While W-18 hasn't been on North America's radar long, it's fast gaining a reputation as a potential grim reaper in a family of synthetic substances known as analogues.

The chemical compounds are very similar to well-known illicit drugs like heroin, with minor differences in structure. And while they copycat illicit drugs, they're still perfectly legal.

Hall said drug dealers order these synthetics and pill presses online, then market them as a legal alternative to most illicit street drugs. The danger with such potent products is that many users, and some dealers, don't know what they're getting.

In Florida, a man was recently sentenced to 10 years in prison for smuggling another deadly synthetic drug, fentanyl. He faced no repercussions for possession of W-18.

A similar situation is brewing in Canada.

Why it's legal

W-18 is not regulated under Canada's Controlled Drugs and Substances Act. ALERT, Alberta's elite policing squad, confirmed to CBC Wednesday that law enforcement cannot bring charges relating to the drug.

Fentanyl is used to treat severe pain with a traditional transdermal patch but is used recreationally via injection, smoking or snorting. (CBC) "That once again shows a reason to explore other ways of banning substances not just by chemical structure but by their pharmaceutical action in the brain," said Hall.

He thinks such a change would hinder chemists who create synthetics with more and more variations that fly outside routine drug tests and the law.

Florida has recently taken such a step, he said, banning substances that mimic the effects of synthetic marijuana, and hallucinogens like bath salts and flakka.

But the caveat with banning opioids like W-18 based on the wider category of their psychological effects is that there are legitimate clinical drugs that have the same impact, used for medical purposes.

While that debate continues, Health Canada is working to add W-18 to its list of controlled substances. It wrapped up public consultations on the issue in mid-March.

Alberta has now asked the federal government to speed up that process.

The provincial health and justice ministers penned an urgent letter the day ALERT's W-18 warning came out, asking their federal counterparts to consider "accelerating the process, given the dangers associated with W-18."

Their message has huge resonance in a province already ravaged by overdoses of drugs that are not commonly known.

Fentanyl rose from obscurity to kill 272 Albertans by overdose in 2015.

W-18 is 100 times more potent than fentanyl.