Butyrfentanyl or butyrylfentanyl is a potent short-acting synthetic opioid analgesic drug.

[1] This document also states that the article describing its clinical effects (analgesic studies, μ-, δ-, κ-opioid receptor binding, and in vitro measures of drug efficacy, antinociceptive, and narcotic properties) was published in 1987.

[2] Butyrfentanyl has no current legitimate clinical applications; however, it is being sold as a designer drug.

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.

[2] High cross-reactivity may be the effect of the shape of the molecule — the shape of butyrfentanyl is closest to the original fentanyl molecule, which makes it easy to bind by fentanyl antibodies.