Fentanyl is a synthetic opioid that is about 50 times stronger than heroin. Pharmaceutical-grade fentanyl is carefully manufactured, regulated, and used in hospital settings or by prescription to treat pain.
Illicit fentanyl, on the other hand, is made without any oversight and can vary in potency based on the maker or the batch. Besides being extremely strong, illicit fentanyl is also relatively inexpensive to get, making it attractive to drug dealers who add it to many other drugs to make them more potent.