Alfentanil (R-39209), sold under the brand name Alfenta among others, is a potent but short-acting synthetic opioid analgesic drug used for anesthesia in surgery.

[3] Alfentanil has a pKa of approximately 6.5, which leads to a very high proportion of the drug being uncharged at physiologic pH, a characteristic responsible for its rapid-onset.

While alfentanil tends to cause fewer cardiovascular complications than other similar drugs such as fentanyl and remifentanil, it tends to give stronger respiratory depression and so requires careful monitoring of breathing and vital signs.

Discovered at Janssen Pharmaceutica in 1976, alfentanil is classified as a Schedule II drug in the United States.

Fentanyl analogs have killed hundreds of people throughout Europe and the former Soviet republics since the most recent resurgence in use began in Estonia in the early 2000s, and novel derivatives continue to appear.