Fentanyl — and its hundreds of chemical cousins — contributed 28,466 deaths. Fifty times stronger than heroin, fentanyl is important as a surgical anesthetic, for treating cancer pain and for end-of-life care. Fentanyl is also a popular street drug, along with derivatives that may be 100 times more potent. Fentanyl drugs are easy to overdose on, and because they are used to cut heroin, result in many overdoses among people who thought they were using heroin, not fentanyl. (Cocaine, marijuana and other drugs may also be adulterated with fentanyl relatives.) Reducing access to prescription opioids could reduce future demand for street opioids.