4-Fluoroisobutyrylfentanyl, also known as 4-FIBF or p-FIBF, belongs to the opioid analgesic class and shares structural similarities with butyrfentanyl. It is a structural isomer of 4-Fluorobutyrfentanyl and has been marketed and distributed online as a synthetic designer drug.
This compound exhibits a solid connection to 4-fluoro fentanyl, with a measured EC50 value of 4.2 nM for the human μ-opioid receptor. Notably, 4-Fluoroisobutyrylfentanyl is an exceptionally selective μ-opioid receptor agonist, and its analgesic potency is nearly ten times greater than that which is typically reported for morphine.

IUPAC name
CAS Number244195-32-2
PubChem CID100974914
Chemical and physical data
Molar mass368.496 g·mol−1

Side effects

The side effects associated with fentanyl analogs closely mirror those of fentanyl, encompassing symptoms such as itching, nausea, and potentially severe respiratory depression, which poses a life-threatening risk. Since the resurgence of their use in Estonia in the early 2000s, fentanyl analogs have resulted in the loss of numerous lives across Europe and the former Soviet republics. Furthermore, novel derivatives of these substances persistently emerge.


In the United States, 4-Fluoroisobutyrylfentanyl is classified as a Schedule I controlled substance. It is categorized as a Class A drug in the United Kingdom, and in Canada, it is designated as a Schedule I drug.


1. What is 4-Fluoroisobutyrfentanyl?

4-Fluoroisobutyrfentanyl, often referred to as 4-FIBF, is an opioid analgesic that is chemically related to fentanyl and is known for its potent pain-relieving effects.

2. Is 4-Fluoroisobutyrfentanyl legal?

No, it is not legal in many countries, including the United States, the United Kingdom, and Canada, where it is classified as a controlled substance or a prohibited drug.

3. What are the common street names for 4-Fluoroisobutyrfentanyl?

Some common street names for this substance include 4-FIBF and p-FIBF.

4. What are the effects of 4-Fluoroisobutyrfentanyl?

The effects are similar to other opioids and may include pain relief, sedation, and potential euphoria. However, it also carries a risk of adverse effects, including respiratory depression and addiction.

5. Are there any medical uses for 4-Fluoroisobutyrfentanyl?

No, it is not approved for medical use and is primarily encountered as a synthetic designer drug.

6. How is 4-Fluoroisobutyrfentanyl administered?

It is typically encountered in the form of a powder or crystals and can be ingested orally, snorted, or even injected by individuals seeking its effects.

7. What are the risks associated with 4-Fluoroisobutyrfentanyl use?

Using 4-fluoroisobutyrfentanyl can lead to various risks, including overdose, respiratory depression, addiction, and potential fatalities.

8. Can 4-Fluoroisobutyrfentanyl be detected in drug tests?

Yes, it can be detected in standard drug tests designed to identify opioid use.

9. How has 4-Fluoroisobutyrfentanyl contributed to public health concerns?

The use of 4-Fluoroisobutyrfentanyl and other fentanyl analogs has led to numerous overdose deaths and public health crises, particularly in regions where they have been prevalent.

10. Is there ongoing research or regulation regarding 4-Fluoroisobutyrfentanyl?

Regulatory authorities and law enforcement agencies continuously monitor and take measures to control and restrict the distribution of such substances. Additionally, research on these analogs is ongoing to understand their effects and potential risks better.


  1. FIBF – Innovations in Synthetic Opioid Research Cayman Chemical is at the forefront of synthetic opioid research, exploring new compounds and their effects. Our team has collaborated with researchers worldwide to advance the understanding of these substances. One significant study was conducted by Watanabe and colleagues in July 2017.
  2. In-Depth Metabolite Identification In the groundbreaking work published in “The AAPS Journal,” Watanabe, Vikingsson, Roman, Green, Kronstrand, and Wohlfarth delved into in vitro and in vivo metabolite identification for novel synthetic opioids. The study focused on compounds such as Acetylfentanyl, Acrylfentanyl, Furanylfentanyl, and 4-Fluoro-Isobutyrylfentanyl.
  3. Swedish STRIDA Project Findings Helander, Bäckberg, Signell, and Beck shared their research results from the Swedish STRIDA project in July 2017. Their study centered on intoxications involving acrylfentanyl and other designer fentanyls, shedding light on the impact of these substances on public health.
  4. Insights into Concentrations Strayer, Antonides, Juhascik, Daniulaityte, and Sizemore’s work, published in January 2018 in “ACS Omega,” highlighted concentrations of these synthetic opioids. This research contributed to our understanding of their chemical properties.
  5. New York City Medical Examiner’s Study Pardi, Toriello, and Cooper, in January 2020, conducted an 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. Their findings provided valuable insights into the prevalence of this synthetic opioid.
  6. Receptor Interactions Ulens, Van Boven, Daenens, and Tytgat, in September 2000, explored the interaction of p-fluorofentanyl with cloned human opioid receptors. Their study investigated the role of specific amino acids in mu-opioid receptor selectivity.
  7. Assessing 4F-iBF Risks A comprehensive “4F-iBF risk assessment” conducted by relevant authorities is available for reference. This assessment provides important information about the potential dangers associated with this synthetic opioid.
  8. The Rising Trend of Fentanyls Mounteney, Giraudon, Denissov, and Griffiths, in July 2015, raised awareness about the increasing prevalence of highly potent fentanyls in Europe. Their study highlighted the urgent need for monitoring and control.
  9. DEA Controlled Substances The Drug Enforcement Administration (DEA) maintains a list of controlled substances by DEA Drug Code Number, which includes these synthetic opioids. This resource plays a crucial role in regulating and controlling these compounds.

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