4-Fluorobutyrylfentanyl, also referred to as 4-FBF or para-fluorobutyrylfentanyl, is a synthetic opioid analgesic. This compound, closely related to buying fentanyl, has gained popularity as a designer drug and is available online. It shares a close chemical kinship with 4-fluoro fentanyl, a substance with a notable affinity for the human μ-opioid receptor, as indicated by its EC50 value of 4.2 nM.

IUPAC name
CAS Number244195-31-1
PubChem CID86280430
Chemical and physical data
Molar mass368.496 g·mol−1

Side Effects

The side effects associated with fentanyl analogs are akin to those observed with fentanyl. These may encompass sensations of itching, bouts of nausea, and, more critically, the potential for severe respiratory depression, which can be life-threatening. Fentanyl analogs have been responsible for numerous fatalities across Europe and the former Soviet republics since experiencing a resurgence in usage, notably originating in Estonia during the early 2000s. This trend continues to give rise to novel derivatives.

Legal status

On August 18, 2014, Sweden’s public health agency recommended categorizing 4-fluorobutyrylfentanyl as hazardous.
In October 2015, China instituted regulatory control over 4-fluorobutyrylfentanyl, designating it as a controlled substance.
As of February 1, 2018, 4-Fluorobutyrfentanyl is a Schedule I controlled substance in the United States.


  • What is 4-fluorobutyrfentanyl?
  • 4-Fluorobutyrfentanyl, also known as 4-FBF, p-FBF, or para-fluorobutyrylfentanyl, is an opioid analgesic. It is structurally related to butyrfentanyl and has been sold online as a designer drug.
  • How does 4-fluorobutyrfentanyl work?
  • Like other opioids, 4-Fluorobutyrfentanyl works by binding to opioid receptors in the brain, primarily the μ-opioid receptor. This binding results in pain relief and, at the same time, may lead to side effects and potential risks.
  • What are the side effects of 4-Fluorobutyrfentanyl?
  • Side effects associated with 4-Fluorobutyrfentanyl use are similar to those of fentanyl and other opioid drugs. These can include itching, nausea, and potentially severe respiratory depression, which can be life-threatening.
  • Has 4-fluorobutyrfentanyl been regulated or banned?
  • Yes, several countries have taken regulatory actions concerning 4-Fluorobutyrfentanyl. For instance, Sweden’s public health agency recommended categorizing it as hazardous in 2014. In October 2015, it became a controlled substance in China, and as of February 1, 2018, it is classified as a Schedule I controlled drug in the United States.
  • Is 4-fluorobutyrfentanyl legal in any country?
  • The legal status of 4-Fluorobutyrfentanyl varies by country. Knowing and complying with the laws and regulations regarding this substance in your location is crucial.
  • Is 4-fluorobutyrfentanyl safe for use?
  • The use of 4-Fluorobutyrfentanyl, like other potent opioids, carries inherent risks, including the potential for overdose and respiratory depression. It is essential to exercise extreme caution and, if necessary, seek medical guidance when dealing with such substances.
  • Why is there concern about designer opioids like 4-fluorobutyrfentanyl?
  • Designer opioids pose risks due to their potency, variability in purity, and the potential for unanticipated side effects. Additionally, their legality may be uncertain, and the lack of regulation makes them dangerous to users.
  • Where can I find more information about 4-Fluorobutyrfentanyl?
  • To stay informed about the latest updates, regulatory changes, and safety recommendations concerning 4-Fluorobutyrfentanyl, consult reputable sources such as government health agencies, academic research, and medical professionals.


  1. “Para-Fluorobutyrylfentanyl”. This information was sourced from Cayman Chemical and is dated back to 15 July 2015.
  2. In a study published on 9 August 2015, Bäckberg M, Beck O, Jönsson KH, and Helander A investigated opioid intoxications, specifically those involving butyrfentanyl, 4-fluorobutyrfentanyl, and fentanyl as part of the Swedish STRIDA project. Their findings were documented in the journal Clinical Toxicology (Volume 53, Issue 7, Pages 609–17) with the following reference: doi:10.3109/15563650.2015.1054505. The PMID for this publication is 26083809, and it is also indexed under S2CID 4071678.
  3. On September 2000, Ulens C, Van Boven M, Daenens P, and Tytgat J explored the interaction of p-fluorofentanyl with cloned human opioid receptors and delved into the role of Trp-318 and His-319 in mu-opioid receptor selectivity. Their research was presented in The Journal of Pharmacology and Experimental Therapeutics (Volume 294, Issue 3, Pages 1024–33) and can be found under the PMID 10945855.
  4. In July 2015, Mounteney J, Giraudon I, Denissov G, and Griffiths P discussed the rising prevalence of fentanyls and the potential signs that were being overlooked in Europe. Their work was published in The International Journal on Drug Policy (Volume 26, Issue 7, Pages 626–31) with a reference to doi:10.1016/j.drugpo.2015.04.003 and the PMID 25976511.
  5. An announcement regarding the classification of new substances as narcotics or hazardous items was made by Folkhälsomyndigheten on 18 August 2015. This information is in Swedish and can be verified by the reference provided.
  6. On 27 September 2015, the China Food and Drug Administration released a notice related to the “Non-Medicinal Narcotic Drugs and Psychotropic Substances Control Method.” This document is in Chinese and is available for reference.
  7. On 1 February 2018, the “Schedules of Controlled Substances” were updated, including the temporary placement of seven Fentanyl-related substances in Schedule I, as reported in the Federal Register.

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