para-Fluorobutyryl fentanyl, also known as PFBF and 4-FBF, is a designer drug and fentanyl analog that is abused recreationally.

para-Fluorobutyryl fentanyl (PFBF):
https://www.cerilliant.com/shopOnline/Item_Details.aspx?itemno=86a457dd-47b2-42da-988d-07cf1d20ebda&item=F-048-0.5ML

World Health Organization 2018:

Substance identification:
p-Fluoro-butyrylfentanyl(IUPAC name: N-(4-fluorophenyl)-N-[1-(2-phenylethyl)piperidin-4-yl]butanamide) is a synthetic analog of the opioid analgesic fentanyl. In Europe, it was first reported in 2014 followed by the Untied States of America in 2015. Samples obtained from seizures and collections suggest that p-fluoro-butyrylfentanyl appearsin powder, tablet, nasal spray and e-liquid form. Examples exist where it might be found in samples mixed with heroin.

General pharmacology:
Pharmacological studies have shown that p-fluoro-butyrylfentanyl is qualitatively similar to fentanyl and heroin.

Abuse potential:
Whilst no formal studies exist, the limitedbavailable information indicates that p-fluoro-butyrylfentanyl is used by experimental users (psychonauts) and people who also use synthetic opioids and opiates. It is likely that p-fluoro-butyrylfentanylwill be associated withabuse liability.

Non-medical use:
The mode of use may involve the combinational use (intentionally or unintentionally) of other drugs and users may be unaware of the exact dose or compound being ingested (by whatever route). Similar to other fentanils, p-fluoro-butyrylfentanyl may be administered as a solution (e.g. using nasal sprays), orally as a powder (including in capsules or tablets), or by insufflation of a powder; it can also be administered sublingually or intranasally via a spray; administered by injection (intramuscular or intravenous) or inhaled by vaporizing.

Critical Review Report: p-Fluoro-butyrylfentanyl (PDF 25 pages) (2018):
https://www.who.int/medicines/access/controlled-substances/Parafluorobutyrfentanyl.pdf

para-fluorobutyryl fentanyl Monograph (PDF 5 pages):
https://swgdrug.org/Monographs/para-Fluorobutyryl%20fentanyl.pdf

para-Fluorobutyryl fentanyl (hydrochloride) Safety Data Sheet (PDF 5 pages):
https://www.caymanchem.com/msdss/17049m.pdf

Also known as 4-FBF and p-FBF - An opioid analgesic that is an analog of butyrfentanyl and has been sold online as a designer drug.

Side Effects:
Side effects of fentanyl analogs are similar to those of fentanyl itself, which include itching, nausea and potentially serious respiratory depression, which can be life-threatening. 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.