Isobutyryl-carfentanyl is the most recently discovered fentanyl analogue with a chemical structure that is similar to that of carfentanyl. Its analogue, carfentanyl, is regarded as one of the most lethal drugs in the world, with a potency of 10,000 times that of morphine. Therefore, isobutyryl-carfentanyl may possess a comparably high potency and its harmful effects cannot be ignored.

This study was designed to assess the analgesic effect of isobutyryl-carfentanyl and the potential risks associated with its misuse.

In this study, we assessed the acute toxicity of isobutyryl-carfentanyl by up-and-down-procedure, the analgesic efficacy by hot-plate test, the abuse potential by conditioned place preference (CPP), drug self-administration, and drug discrimination tests, and compared it with fentanyl and carfentanyl.

The estimated median lethal dose (LD50) of isobutyryl-carfentanyl administered were 175 mg/kg (intragastric administration, IG), 15.84 mg/kg (intraperitoneal injection, IP), 15.84 mg/kg (subcutaneous injection, SC), and 1.6 mg/kg (intravenous injection, IV), respectively. The 50% maximal analgesic effect (ED50) of isobutyryl-carfentanyl was determined to be 0.00319 mg/kg, with an analgesic potency 14 times that of fentanyl and 0.82 times that of carfentanyl. Isobutyryl-carfentanyl exhibited a significant positional preference at a minimum dose of 0.1 mg/kg, while fentanyl exhibited a significant positional preference at a minimum dose of 0.3 mg/kg. In the heroin (0.05 mg/kg/infusion) self-administration substitution experiment, isobutyryl-carfentanyl showed significant self-administration behaviour at doses of 0.0005–0.001 mg/kg/infusion, with the maximum number of infusions observed at a dose of 0.001 mg/kg. In the heroin (1 mg/kg) drug discrimination experiment, fentanyl (0.005–0.02 mg/kg), carfentanyl (0.0005–0.002 mg/kg), and isobutyryl-carfentanyl (0.001–0.005 mg/kg) were tested in the dose-effect curves. The results showed that all three drugs exhibit dose-dependent increase in the number of drug-associated nose pokes responses and reduction in the rate of nose pokes. The subjective effect potency of isobutyryl-carfentanyl was found to be 4.4 times that of fentanyl and 0.5 times that of carfentanyl.

In summary, isobutyryl-carfentanyl has high acute toxicity and analgesic effect, with strong psychological dependence approximately 5 times that of fentanyl and 0.5 times that of carfentanyl, and has extremely high abuse potency.