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Doctor holding a pill containerAndrey Popov/Getty Images/iStockphoto

For the past few months, Canadians have been hearing more about the dangers of W-18, a so-called synthetic opioid 100 times more powerful than fentanyl. The concern about this drug is so great the federal government added it earlier this month to the Controlled Drug and Substances Act, which makes it illegal to produce, possess, import, export or traffic W-18.

The move "will help law enforcement across Canada keep this dangerous substance off of Canadian streets, and out of the hands of vulnerable individuals," according to a Health Canada news release.

But the language in a new advisory issued by Health Canada on Tuesday strikes a much different tone.

The department said it wanted to "clarify" its position on W-18, which it described as a "potentially harmful substance."

So what's going on?

It turns out that Health Canada's earlier statements describing W-18 as an opioid much more potent and dangerous than fentanyl were premature.

The drug, which was developed in the 1980s at the University of Alberta but was never marketed commercially or approved for sale, may pose a public health threat. But the department said there are too many unanswered questions to make any clear statement about its potential dangers.

Health Canada stated that "doubts about this classification" of W-18 as a potent opioid have been raised "by a number of credible sources." One of the key questions that has been raised is whether W-18 is, in fact, an opioid. According to Health Canada's new statement, there is no scientific evidence showing that W-18 binds to opioid receptors in the body.

Without hard evidence, it's difficult to say, with any authority, what the risks of W-18 might be.

What we do know, from a patent application, is that a study on mice showed W-18 was more effective at relieving pain than morphine. According to Health Canada, the drug was active at 1/10,000 of the dose compared to morphine, which could imply it could be highly toxic in humans.

But several experts have pointed out that this doesn't prove anything about W-18 and that more research is needed to truly understand the potential dangers.

The department will "continue to assess new information as it becomes available" about how W-18 works. In the meantime, it is also advising the public that naloxone, which is used to treat people suffering from an opioid overdose, might not be effective at counteracting the effects of W-18.