Philadelphia health officials are issuing a warning about a lethal heroin mix that has entered the city's illegal drug market.

Since the beginning of the year, 28 overdose deaths -- almost half of the city's total deadly overdoses -- have been linked to heroin laced with an opioid called fentanyl. It's a powerful painkiller often used to treat cancer pain.

"Fentanyl is about 50 or 60 times stronger than morphine or heroin, so this medication can be extremely dangerous," explained Matthew Hurford, chief medical officer for the city's department of behavioral health.

Health officials are trying to get the word out about the risks by distributing an alert.

"We are notifying our provider community, we are notifying hospitals," said Roland Lamb, who heads addiction services for the department of behavioral health.

"Even though they may have been hearing about it in the suburbs, it is now right in Philadelphia, and we need to be aware of it," he said.

The goal is to avoid more overdose deaths, said Lamb, and to perhaps push addicts into seeking treatment because of the increased danger of using heroin right now.

The alert also communicates changes in treatment approaches in emergency-medicine settings, Hurford said.

"It can help them think about fentanyl overdose sooner in their treatment, when they suspect that somebody may have overdosed on an opiate, and that can help them make the right interventions sooner to save lives," he said.

When used in excess, Hurford said, fentanyl can shut down a person's breathing, and it requires higher doses of opioid antidotes than heroin.