A pharmacy in southwestern Ontario put fentanyl -- a powerful opioid linked to numerous overdose deaths -- inside a kit that was supposed to contain its antidote, naloxone.

The potentially disastrous mix-up occurred on Feb. 5 at a Shoppers Drug Mart location in the quiet community of Elmira, near Waterloo, Ont.

The exact location of the improper kit was initially unknown. On Saturday, CTV Kitchener confirmed that the kit was sold in Elmira.

The Ontario College of Pharmacists has launched an investigation.

“The pharmacy is fully cooperating and we are confident that immediate action has been taken to begin to determine how this happened and how it could have been prevented,” the regulatory body’s spokesperson, Todd Leach, said in a written statement.

“We have not been made aware of any similar incidents occurring at other pharmacies.”

Administered via nasal spray or injection, naloxone is used to reverse the effects of opioid overdoses. It is available free of charge at numerous Canadian pharmacies.

Loblaw Companies Limited, which acquired the Shoppers Drug Mart pharmacy chain in 2013, called the incident “a case of human error” that “absolutely should not have happened.”

“Our local pharmacist-owner took immediate action to ensure consumer safety, visiting the customers to secure the product and provide the correct medication, and to offer a detailed apology,” company spokesperson Catherine Thomas said via email.

“We are taking appropriate steps to reinforce professional and operational expectations and procedures to help prevent this type of unfortunate situation from reoccurring.”

Thomas added that the error was an “isolated event” that she hopes does not reduce customers’ confidence in naloxone kits and their ability to save lives.

In nearby Guelph, Ont., Kenneth Chan, the pharmacist-owner of University Square Pharmacy, was shocked that such an error could occur. At his pharmacy, naloxone kits come preassembled while fentanyl is kept in a completely separate area under lock and key.

“Wherever this pharmacy obtained their naloxone kit, either they made a mistake there,” he hypothesized, “or when they assembled the kit themselves, something happened.”

With files from CTV Kitchener’s Heather Senoran, CTV Toronto’s Chris Fox and CP24