4-fluorobutyrfentanyl 4-fluorobutyrfentanyl 4-fluorobutyrylfentanyl (also known as 4-fbf and p-fbf or para-fluorobutyrylfentanyl) is an opioid analgesic that is an analog of butyrfentanyl and has been sold online as a designer drug. it is closely related to 4-fluorofentanyl, which has an ec50 value of 4.2 nm for the human μ-opioid receptor. 4-fluorobutyrfentanyllegal statuslegal status ca: schedule i de: anlage ii (authorized trade only, not prescriptible) uk: class a us: schedule i illegal in china and sweden identifiers iupac name n-(4-fluorophenyl)-n--butanamide cas number244195-31-1pubchem cid86280430chemspider32779976unii2378l0e82tchemical and physical dataformulac23h29fn2omolar mass368.496 g·mol−13d model (jsmol)interactive image smiles cccc(=o)n(c1ccn(cc1)ccc2=cc=cc=c2)c3=cc=c(c=c3)f inchi inchi=1s/c23h29fn2o/c1-2-6-23(27)26(21-11-9-20(24)10-12-21)22-14-17-25(18-15-22)16-13-19-7-4-3-5-8-19/h3-5,7-12,22h,2,6,13-18h2,1h3key:qzfmcyubpslobp-uhfffaoysa-n 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. legal status sweden's public health agency suggested classifying 4-fluorobutyrylfentanyl as hazardous substance on august 18, 2014. in october 2015, 4-fluorobutyrylfentanyl became a controlled substance in china. 4-fluorobutyrfentanyl is a schedule i controlled drug in the usa since 1. february 2018.