Fentanyl is the most widely used synthetic opioid in medicine. The compound is a potent pain reliever and is also used for anesthesia (along with other medications). Fentanyl is considered safe and effective when used as directed by a physician. However, fentanyl is highly addictive, more than 20,000 deaths in the U.S. in 2016 were attributed to overdoses of fentanyl and its derivatives.