A veterinary expert says he's "horrified" a drug used to sedate large animals like elephants, rhinos and bison has led to the deaths of two men in Alberta.

Earlier this month, traces of the opioid carfentanil were found during autopsies of two men in their 30s from Edmonton and Calgary.

Carfentanil is 100 times more potent than fentanyl, a drug that has already killed 153 people in the first half of this year. 

As head of the Department of Veterinary Clinical and Diagnostic Science in the Faculty of Veterinary Medicine at the University of Calgary, Nigel Caulkett is familiar with carfentanil. He's used it to immobilize large wild animals.

Caulkett said veterinarians would wear full safety equipment when handling the drug, and in many zoos a paramedic had to be present when it was being used.

Nigel Caulkett

Nigel Caulkett says one drop of the carfentanil veterinarians used was equal to six times the lethal dose of morphine.

Less potent alternatives to carfentanil are becoming more commonly used by veterinarians, he said. 

"The reason we use carfentanil — or used to, we can no longer get it — is because I can use one millilitre, or one small dart of carfentanil to anaesthetize a full grown male bison," Caulkett said.

"One drop of the carfentanil we used to work with was equal to six times the lethal dose of morphine."

"Those of us that work with it know it's an extremely potent and powerful drug, so the thought of it being on the street is quite scary." - Nigel Caulkett, veterinarian

Having handled the drug, Caulkett was shocked humans were using it, but says it's not the first time in his field that they've lost drugs used for wildlife capture due to human abuse. 

"I was fairly horrified, my colleagues and I found out a few months ago that carfentanil had hit the streets in the U.S. and we were wondering when it might reach Canada," he said.

"Those of us that work with it know it's an extremely potent and powerful drug, so the thought of it being on the street is quite scary."

Alberta Health says dealers are adding it to traditional drugs, sometimes without telling their users.

Alberta's acting chief medical examiner, Dr. Elizabeth Brooks-Lim, said one of the men who died also had fentanyl in his system.

The drug resembles table salt, and Caulkett said he believes that and its potency is why it has ended up on the street.

Caulkett said he hopes Alberta Health sounding the alarm will be enough to warn people about the drug.

"Given the extreme potency, the chance of mixing it up improperly, the difficulty reversing it, the amount of antagonist that's required in the field and also the duration, it's a very deadly drug."