A drug testing service in Toronto has come across a potent new animal tranquilizer in the city's illicit drug supply.

Toronto's Drug Checking Service (TDCS), which is a free and confidential health service provided at five harm reduction agencies, announced on Jan. 29 that they had first identified a substance that could be either medetomidine or dexmedetomidine last month.

The substance was detected in 15 out of 140 samples tested between Dec. 29 and Jan. 23. All samples were collected from the downtown or west end and believed to be fentanyl.

Medetomidine is an animal tranquilizer, approved only for use on animals including dogs, while dexmedetomidine is approved for use in both humans and animals for sedation and pain relief.

The two anesthetic drugs have very similar chemical structures. The service can't currently differentiate between them, according to the TDCS.

"The presence of medetomidine/dexmedetomidine wasn't anticipated by those who submitted these samples for testing," according to the service.

"We suspect medetomidine/dexmedetomidine is being mixed with unregulated fentanyl to mimic or enhance the sedative and euphoric effects of the opioid a person chooses to use."

This discovery underscores how new, dangerous additives continue to appear in Canada's highly toxic drug supply, increasing the risk of overdose and other adverse health consequences for people who use drugs.

CBC News has previously reported that xylazine, a potent veterinary sedative, is being mixed with opioids like fentanyl to extend their effects.

Xylazine is usually used as a sedative in large farm animals like cows and horses, but is not approved for use in humans in Canada and its long-term effects on human health are unknown. As a central nervous system depressant, it dangerously reduces crucial vital signs such as heart rate and blood pressure.

When mixed with fentanyl, it's known as "tranq" or "zombie dope," and can cause long-lasting blackouts and terrible, painful wounds that can lead to amputation.

A March 2023 report from Health Canada showed the rapid spread of xylazine across the country in recent years, with a growing number of street drug samples seized by police testing positive for the tranquilizer, predominantly in Ontario.

Medetomidine/dexmedetomidine are even more potent than xylazine because they last longer and provide deeper sedation, according to the TDCS.

Those who use them might enter a "deep state of unconsciousness" and the risk of severe sedation and drowsiness increases when medetomidine/dexmedetomidine is used along with high-potency opioids, benzodiazepine-related drugs, and xylazine.

"This is noteworthy because 100% of the samples checked by Toronto's Drug Checking Service that contained medetomidine/dexmedetomidine contained at least one high-potency opioid," the service said.

"Many of these samples also contained a benzodiazepine-related drug or xylazine."

Tranquilizers like xylazine and depressants like benzodiazepines (benzos) are highly sedative and pose a particular risk when people are unaware they're consuming them.

Naloxone, a medication used to reverse opioid overdoses, is not effective on these drugs because they aren't opioids, according to the TDCS.

The TDCS advises drug users to minimize potential harm by checking drugs before using, using at supervised consumption centers, using with others or informing others if you're using alone, and calling 911 in an overdose situation.

Toronto Public Health stated that the city continues to grapple with a drug toxicity crisis and calls for increased federal and provincial investments in prevention, harm reduction, and treatment supports.

"Overdoses and deaths attributable to the toxic drug supply can be prevented," TPH said.