Before Justin Gill began working at NorWest Community Health Centres (NWCHC), he thought fentanyl was the most toxic substance impacting drug users in Thunder Bay, Ont.

But he was wrong.

Using the organization's drug analyzer machine, he found traces of carfentanil — which is 100 times stronger than fentanyl — and nitazenes in the substances people brought in.

"That was very alarming," he said. "People think they're getting one thing but there's multiple things in their substance that can kill the…