3-Methyl-thiofentanyl is an opioid

Opioid

An opioid is a psychoactive chemical that works by binding to opioid receptors, which are found principally in the central and peripheral nervous system and the gastrointestinal tract...

 analgesic

Analgesic

An analgesic is any member of the group of drugs used to relieve pain . The word analgesic derives from Greek an- and algos ....

 that is an analogue

Analog (chemistry)

In chemistry, a structural analog , also known as chemical analog or simply analog, is a compound having a structure similar to that of another one, but differing from it in respect of a certain component. It can differ in one or more atoms, functional groups, or substructures, which are replaced...

 of fentanyl.

3-Methyl-thiofentanyl was sold briefly on the black market in the early 1980s, before the introduction of the Federal Analog Act

Federal Analog Act

The Federal Analog Act, , is a controversial section of the United States Controlled Substances Act which allowed any chemical "substantially similar" to a controlled substance listed in Schedule I or II to be treated as if it were also listed in those schedules, but only if intended for human...

 which for the first time attempted to control entire families of drugs based on their structural similarity rather than scheduling each drug individually as they appeared.

3-Methyl-thiofentanyl has similar effects to fentanyl. Side effects of fentanyl analogues are similar to those of fentanyl itself, which include itching, nausea

Nausea

Nausea , is a sensation of unease and discomfort in the upper stomach with an involuntary urge to vomit. It often, but not always, precedes vomiting...

 and potentially serious respiratory depression which can be life-threatening.