One person has been arrested after a suspected supply of tainted drugs led to more than a dozen overdoses in the city, but Toronto police say they can't guarantee the substance is off the streets.

Emergency crews were called to the supervised injection site The Works on Victoria Street site near Dundas Street East and Yonge Street Wednesday evening amid reports of a contaminated drug supply in the city.

The site reversed 16 overdoses within the space of just hours, Toronto board of health chair Joe Cressy said. Twelve people had to be taken to hospital, police say.

For the city's board of health chair, the incident underscores just how important supervised consumption sites are.

"In the face of a toxic drug supply, these sites continue to save lives," Cressy said.

Police say a 38-year-old Toronto man was taken into custody very near to the site and an investigation is underway. He has since been charged with numerous drug-related offences including two counts of possession for the purpose of trafficking and possession of the proceeds of crime.

It's possible that drugs from the same batch are still being sold elsewhere, they say.

"Purchasing, ingesting street drugs truly is a form of Russian roulette. The risk to public safety from ingesting street drugs remains critically high," said Supt. Steve Watts.

Officials are also investigating to find out precisely what substances the drug contained, they say.

Despite being administered naloxone, police say none of the individuals affected recovered in a way that was typical for a normal fentanyl overdose.

That leads police to believe that the substance was a mixture of fentanyl and something else.

On Wednesday, Halton police reported a new substance had been detected in the region containing a benzodiazepine derivative, something naloxone can't effectively treat.

Investigators in Toronto say this city could be dealing with the same or a similar mixture, but that Health Canada will ultimately determine what the substance was. A sample of the supply has also been sent to a laboratory at St. Michael's Hospital for testing.

"The events that unfolded yesterday clearly demonstrate the need for supervised consumption services in our community and the life-saving benefit they provide," said the city's medical officer of health Eileen de Villa.