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P-fluoroisobutyrylfentanyl 3D BS.png
Legal status
Legal status
  • N-(4-Fluorophenyl)-N-[1-(2-phenylethyl)-4-piperidinyl]-isobutanamide
CAS Number
PubChem CID
Chemical and physical data
Molar mass368.496 g·mol−1
3D model (JSmol)
  • CC(C)C(=O)N(C1CCN(CC1)CCC2=CC=CC=C2)C3=CC=C(C=C3)F
  • InChI=1S/C23H29FN2O/c1-18(2)23(27)26(21-10-8-20(24)9-11-21)22-13-16-25(17-14-22)15-12-19-6-4-3-5-7-19/h3-11,18,22H,12-17H2,1-2H3

4-Fluoroisobutyrylfentanyl (also known as 4-FIBF and p-FIBF) is an opioid analgesic that is an analog of butyrfentanyl and structural isomer of 4-Fluorobutyrfentanyl and has been sold online as a designer drug.[1][2][3][4][5] It is closely related to 4-fluorofentanyl, which has an EC50 value of 4.2 nM for the human μ-opioid receptor.[6] 4-fluoroisobutyrylfentanyl is a highly selective μ-opioid receptor agonist whose analgesic potency is almost ten times of that reported for morphine.[7]

Side effects[edit]

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.[8]


4-Fluoroisobutyrylfentanyl is a Schedule I controlled substance in the United States.[9] In the UK, it is a Class A drug. In Canada, it is a Schedule I drug.

See also[edit]


  1. ^ "FIBF". Cayman Chemical.
  2. ^ Watanabe S, Vikingsson S, Roman M, Green H, Kronstrand R, Wohlfarth A (July 2017). "In Vitro and In Vivo Metabolite Identification Studies for the New Synthetic Opioids Acetylfentanyl, Acrylfentanyl, Furanylfentanyl, and 4-Fluoro-Isobutyrylfentanyl". The AAPS Journal. 19 (4): 1102–1122. doi:10.1208/s12248-017-0070-z. PMID 28382544. S2CID 207056734.
  3. ^ Helander A, Bäckberg M, Signell P, Beck O (July 2017). "Intoxications involving acrylfentanyl and other novel designer fentanyls - results from the Swedish STRIDA project". Clinical Toxicology. 55 (6): 589–599. doi:10.1080/15563650.2017.1303141. PMID 28349714. S2CID 11978059.
  4. ^ Strayer KE, Antonides HM, Juhascik MP, Daniulaityte R, Sizemore IE (January 2018). "-1 Concentrations". ACS Omega. 3 (1): 514–523. doi:10.1021/acsomega.7b01536. PMC 5793031. PMID 29399650.
  5. ^ Pardi J, Toriello A, Cooper G (January 2020). "Evaluation of 4-fluoroisobutyrylfentanyl in blood samples from 247 authentic cases submitted to the New York City Office of Chief Medical Examiner in 2017–2018". Forensic Toxicology. 38 (2): 340–351. doi:10.1007/s11419-019-00518-6. ISSN 1860-8973. S2CID 210135027.
  6. ^ Ulens C, Van Boven M, Daenens P, Tytgat J (September 2000). "Interaction of p-fluorofentanyl on cloned human opioid receptors and exploration of the role of Trp-318 and His-319 in mu-opioid receptor selectivity". The Journal of Pharmacology and Experimental Therapeutics. 294 (3): 1024–33. PMID 10945855.
  7. ^ "4F-iBF risk assessment" (PDF). EMCDDA.
  8. ^ Mounteney J, Giraudon I, Denissov G, Griffiths P (July 2015). "Fentanyls: Are we missing the signs? Highly potent and on the rise in Europe". The International Journal on Drug Policy. 26 (7): 626–31. doi:10.1016/j.drugpo.2015.04.003. PMID 25976511.
  9. ^ "Controlled Substances by DEA Drug Code Number" (PDF). Drug Enforcement Administration.