Toronto police are sounding the alarm after a quantity of fentanyl, a powerful opioid that's been linked to hundreds of overdose deaths across Canada, was stolen from a car.

Police say about 20 fentanyl patches, which contain a clear gel that's intended to be time-released through the skin, were stolen from a car in the Yonge Street and Finch Avenue area on Tuesday. Police are concerned those patches may be altered into a lethal form on the street.

"If the patch is cut or altered from its original state and the gel is consumed, the results could be fatal," police said in a news release about the stolen drugs.

Fentanyl, which also comes in pill form, is a powerful prescription painkiller that's far more potent than morphine and heroin. On the street, it goes by the name "little green monster," among others.

The biggest concern is that fentanyl is being added to other drugs like cocaine frequently taken by recreational drug users.

"They don't know how much they're using and they can very easily end up in an overdose situation," said Matthew Young, a senior researcher with the Canadian Centre on Substance Abuse.

A new study released by the substance abuse watchdog says there were 655 fentanyl-related deaths 2009 and 2013, with 472 of those deaths taking place in Ontario.

The study also found the number of overdoses appeared to be increasing year after year.

In B.C., there were 16 fentanyl overdoses last Sunday alone.