Heroin laced with the narcotic painkiller fentanyl is being blamed for at least 28 deaths in Philadelphia from March 3 to April 20, prompting city officials to issue a public warning.

Fentanyl - a synthetic opioid analgesic with a rapid onset and short active duration - is reportedly 50 times stronger than morphine. By suppressing respiration receptors in the brain, fentanyl can stop a user's breathing and cause death.

Philly officials hope this trend doesn't equal the all-out epidemic it became in 2006, when heroin laced with fentanyl killed 269 people in the city. Over 2,000 deaths across the nation were attributed to the killer combo that year.   

Fentanyl can be produced in white powder form making it difficult to distinguish from heroin. City officials have yet to determine who is manufacturing the illegal fentanyl and where it is coming from; in previous cases it was synthesized outside the US.

The 28 dead in Philadelphia ranged in age from 16 to 66, with 60 percent of the victims being white males. As of last week, officials were awaiting test results on seven more overdoses.