The heroin- like drug rots the skin from the inside out similar to flesh-eating bacteria.
Three patients have been treated this week at Saint Joseph Medical Center in Joliet.
The name comes from the scaly green appearance of a user's skin.
"The average life expectancy of someone using crocodile is less than two years," said Dr. Abhin Singla, St. Joseph Medical Center.
Doctors say the drug is 3 times more potent than heroin and costs just $8 a dose.