Photo: The Canadian Press

Testing for the presence of fentanyl.

Toronto police have issued a public safety alert following four suspected drug overdose deaths since Thursday, something they call highly unusual.

Toronto Const. Craig Brister says the concern is the possibility of heorin laced with fentanyl, but investigators are still waiting for tests to come back.

Police say there have been a total of 20 overdoses in the city's downtown core in a three day period.

Investigators say the most recent death occurred Saturday when a woman was found dead in a stairwell.

Police said she died of drug-related causes, but weren't sure if the fentanyl-laced heroin is to blame.

Police say anyone coming into contact with a substance suspected to be heroin should exercise extreme caution and should contact police immediately if something is spotted in parks or public areas.

Many Canadian cities have grappled with drug overdose deaths in recent months, such as Vancouver, which recorded 25 deaths and nearly 600 overdose calls last month alone.

More than 1,000 people died of overdoses in British Columbia last year and the coroner says the powerful opioid fentanyl was involved in many of those deaths.