Thenylfentanyl is an analogue of fentanyl where the phenethylamine side-chain has been replaced by a thiophenylmethyl group.

The legality of fentanyl analogues vary by country.

In much the same way that benzylfentanyl can be used to synthesize fentanyl, thenylfentanyl could ostensibly be used for the same purpose.

The substance is also a structural isomer of acetylthiofentanyl, wherein a methyl group on the thiophene sidechain has been relocated to the carboxamide group nearest the aniline.

It is due to this reduced sidechain length that Thenylfentanyl has dramatically diminished MOR activity compared to other fentanyl analogues.