A Philadelphia hospital is reporting a spike in the number of patients who are being treated in the emergency room for overdoses from crack cocaine laced with fentanyl. Experts say fentanyl is being mixed with a number of drugs, including heroin, cocaine and ketamine.

In a recent four-day period, the Hospital of the University of Pennsylvania treated 18 patients for an apparent overdose of crack cocaine laced with fentanyl, HealthDay reports. Three of the patients died from their overdose, the researchers report in this week’s New England Journal of Medicine.

Law enforcement is increasingly findings fentanyl in the drugs they seize, according to Emily Feinstein, Executive Vice President of the Center on Addiction. “Fentanyl is so cheap, and the drug cartels have great access to it. It is a tremendously effective filler because it’s highly addicting. If it doesn’t kill them, people get an intense reaction that creates addiction. It’s cheap, it has a strong high and it makes people come back for more.”