A new type of fentanyl produced overseas and sold via the internet is making overdoses more difficult to treat in North American emergency departments.

Called acryl fentanyl, the drug is resistant to Narcan and often requires multiple doses of the overdose drug to counteract its effects. In some cases, Narcan isn't effective at all against the synthetic opioid.

Based on news reports, acryl fentanyl has been seen in multiple hospital emergency rooms across the country. It's becoming a problem in the Chicago area where the Cook County medical examiner has issued warnings to first responders who may encounter overdose victims.

The problem has been growing for the past two years in Chicago. In 2015, 649 people died due to an opiate overdose in Cook County, Illinois. In 2016, 1,091 people died, an increase of 68 percent. More than half of those deaths involved fentanyl.

The numbers are more alarming when it comes to acryl fentanyl. Last year in Cook County, seven deaths were attributed to the drug. In 2017, between January and April 8, 44 acryl fentanyl deaths occurred.

"Just one dose can easily stop a person from breathing, causing immediate death," Dr. Ponni Arunkumar, Cook County's chief medical examiner, told NBC News.

Synthetic opioids like fentanyl are thousands of times stronger than street heroin and therefore much more likely to cause overdose fatalities. There's no medicinal use for acryl fentanyl, but it's not technically prohibited by law. The Drug Enforcement Administration treats acryl fentanyl as a fentanyl analog under existing regulations.

"In many cases, one dose of naloxone, the heroin antidote, will revive a person who has overdosed on heroin," said Dr. Steve Aks, emergency medicine physician and toxicologist at the Cook County and Hospitals System's Stroger Hospital. "But we are seeing people in our emergency department who need increased doses of naloxone — in some cases as many as four doses — for the patient to be stabilized after ingesting fentanyl, or a heroin/fentanyl combination."

Fentanyl variants are also being seen in Canadian hospitals. Dr. Perry Kendall who leads British Columbia's opiate task force, says multiple versions of the synthetic opiate on the streets of Vancouver.

Earlier this year, carfentanil was found in Vancouver, and more recently acryl fentanyl was discovered in to be in use. Both drugs require multiple doses of antidotes to stop an overdose.

"One treatment lasts for a little while and then they go down again because the pharmokinetic nature of some of these analogues are different and they last longer [than fentanyl]," Kendall told CBC News.

Part of the problem is determining what drug people are using, says a community advocate working to prevent overdoses.

"We really don't know what people are using. We know that sometimes a week will go by and it will be really strong, and it will take a long time and a lot of Narcan to get people to come back from the overdose," said Sarah Blythe with the Overdose Prevention Society.

The Vancouver Police Department says there were 141 overdose deaths in the city between January and May 15 in the last week of April alone.