Chicago public health officials are advising Lollapalooza goers to test their drugs before partaking to make sure they are not laced with fentanyl.
The Chicago Department of Public Health put out a warning a day before the four-day music festival was set to begin in Grant Park.
"ATTN Chicago & LOLLA FANS," a tweet from CDPH reads. "Fentanyl is a strong opioid that can easily cause overdose. Test your drugs before you use, carry Narcan, and don’t use alone. Call 911 if you suspect someone is overdosing."
CDPH said fentanyl is found in cocaine, MDMA and other drugs – not just heroin. The department also said Narcan and fentanyl test strips can be picked up at Chicago public libraries "in easy-to-access Narcan dispenser kits."
Narcan, also known as Naloxone, is a drug used to help to reverse symptoms of an overdose and save lives. Opioid overdoses typically include a drop in blood pressure, slowed heart rate, slowed or stopped breathing, blue lips and pale skin.
Fentanyl is a synthetic opioid behind a massive spike in overdose deaths across the U.S. in recent years. Just a 2 mg dose is enough to kill a user with zero tolerance, whereas 60 mg of heroin is considered a lethal dose.
Chicago alone saw close to 1,300 fatal fentanyl poisonings last year. The DEA warned last week that fentanyl would almost assuredly be present at the Lollapalooza festival.
"There's going to be people walking around with backpacks at Lollapalooza, that are going to be selling pills that are going to be laced with fentanyl. There's not a doubt in my mind," Shane Catone of the Chicago DEA told FOX 32. "Lollapalooza is like Christmas for drug traffickers."
The annual music festival begins Thursday and ends Sunday, July 31.
Fox News' Andrew Mark Miller contributed to this report.