Fentanyl is a potent opioid which is used for pain relief and anesthesia. It has existed since the 1960s and non-medical use has occurred since the 1970s.
The drug has often been misrepresented as other opioids (e.g. heroin and oxycodone), resulting in many deaths. Its potency makes the drug more difficult to dose correctly when you aren’t taking a pharmaceutical product.
It offers a range of recreational effects, though it tends to provide less euphoria than other opioids.