Fentanyl is a synthetic opioid typically used to treat patients with chronic severe pain or severe pain following surgery. Fentanyl is a Schedule II controlled substance that is similar to morphine but about 100 times more potent. Under the supervision of a licensed medical professional, fentanyl has a legitimate medical use. Patients prescribed fentanyl should be monitored for potential misuse or abuse. The fact sheet addresses how fentanyl affects the body, effects of an overdose, resources, treatment, etc.