Introduction: In this study, the encapsulation of fentanyl citrate as an opioid drug with hydrophobic nature in the nanostructured lipid carrier (NLC) is performed. Methods: For encapsulation of fentanyl citrate drug, hot homogenization method is used. The pharmacokinetics of encapsulated fentanyl citrate for pain relief of rats are investigated. The influence of important variables such as the ratio of liquid lipid to the total amount of lipids, surfactant type and concentration on the particle size is investigated using response surface method. Results: Results show that the optimal NLC size is about 90 nm with PDI value around 0.2 and zeta potential of −25±4.01 mV. Characterization analysis of optimal nanostructure shows successful encapsulation of the drug in nanostructure with a spherical morphology of the NLC structure. Results of drug release from commercial fentanyl citrate ampoule and NLC form indicate a control drug release from the NLC within 72 hours in comparison to the commercial ampoule. In vivo studies show that fentanyl citrate-loaded NLC not only has the potential to relieve pain in doses equal to commercial drug but also it can reduce the dose of the drug about 50%. Conclusion: In conclusion, NLC form of fentanyl citrate can increase the efficacy of the drug by appropriate drug distribution in the body and can reduce the risks of overdose.

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