TORONTO—By his own account, Leon “Pops” Alward should be dead. The 47-year-old Toronto resident has overdosed five times in the past four years after injecting the synthetic opioid fentanyl. He is still alive today because in four of those five emergencies, he received timely shots of naloxone, an opioid antidote.

Mr. Alward, like any Ontario resident, can get as much naloxone as he needs free from one of the more than 2,800 pharmacies that stock the medication in the province. The federal government made the drug available...