In a 24-hour span the Guelph Community Health Centre says there were 12 overdoses in the city.
They all happened from Thursday afternoon to Friday afternoon.
Fentanyl is suspected in all 12 cases, the centre says.
Three of the overdoses happened at its consumption and services treatment site.
They do not believe anyone has died from the overdoses, but they were not certain.
Those who use the health centre say it saves lives.
"You increase your chance of survival by at least 90 per cent," says Matthew Delaney, who is addicted to opioids.
The executive director at the centre says they've reversed around 70 overdoses in just over a year.
Three other people overdosed on white fentanyl two weeks ago, prompting an alert from public health.
Police have not said whether the latest incidents were also connected to that batch.