Abstract: Objective To investigate the effects of fentanyl citrate on pain in aged rats with hip fracture and its underlying mechanisms. Methods Thirty aged rats with hip fracture were divided into model group (n=10), fentanyl citrate group (n=10), and fentanyl citrate + Sar-SP group (n=10). Additionally, healthy rats from the same period were included as normal group (n=10).Rats in the fentanyl citrate group received intravenous tail injection of 10 μg/kg fentanyl citrate, while those in the fentanyl citrate + Sar-SP group underwent intrathecal injection of 10 μL Sar-SP for fentanyl citrate intervention. Rats in the normal and model groups received intravenous tail injection of an equal volume of saline. Pain threshold, weight-bearing capacity, temperature, and dorsoventral thickness of the hind paws were measured before treatment (0 h) and at 1, 6, 24, and 168 h post-treatment. Enzyme-linked immunosorbent assay (ELISA) was used to detect levels of tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), interleukin-1β (IL-1β), and glutamic acid (Glu) in the spinal cord tissue of rats. Immunohistochemical staining was employed to observe the positive expression rates of ionized calcium-binding adapter molecule 1 (Iba-1), glial fibrillary acidic protein (GFAP), and substance P (SP) in the spinal cord tissue. Real-time fluorescent quantitative polymerase chain reaction (qRT-PCR) was conducted to detect the relative expression levels of SP and tachykinin receptor 1 (TACR1) mRNA in the hippocampus tissue of rats. Western blot analysis was performed to assess the relative expression levels of vimentin (VIM), nuclear receptor corepressor 1 (NOCR), ciliary neurotrophic factor receptor (CNTFR), and epidermal growth factor receptor (EGFR) proteins in the spinal cord tissue. Results Compared with the normal group, rats in the model group exhibited decreased pain threshold and weight-bearing capacity, increased hind paw temperature and dorsoventral thickness, as well as elevated levels of TNF-α, IL-6, IL-1β, and Glu, increased positive expression rates of Iba-1 and GFAP, heightened positive expression rate and relative mRNA expression levels of SP and TACR1, and augmented relative protein expression levels of VIM, NOCR, CNTFR, and EGFR in the spinal cord tissue at 0 h before treatment and at 1, 6, 24, and 168 h post-treatment (P<0.05). Compared with the model group, rats in the fentanyl citrate group showed increased pain threshold and weight-bearing capacity, decreased hind paw temperature and dorsoventral thickness, as well as reduced levels of TNF-α, IL-6, IL-1β, and Glu, decreased positive expression rates of Iba-1 and GFAP, lowered positive expression rate and relative mRNA expression levels of SP and TACR1, and decreased relative protein expression levels of VIM, NOCR, CNTFR, and EGFR in the spinal cord tissue at 1, 6, 24, and 168 h post-treatment(P<0.05). Compared with the fentanyl citrate group, rats in the fentanyl citrate+Sar-SP group demonstrated decreased pain threshold and weight-bearing capacity, increased hind paw temperature and dorsoventral thickness, elevated levels of TNF-α, IL-6, IL-1β, and Glu, increased positive expression rates of Iba-1 and GFAP, heightened positive expression rate and relative mRNA expression levels of SP and TACR1, and elevated relative protein expression levels of VIM, NOCR, CNTFR, and EGFR in the spinal cord tissue at 1, 6, 24, and 168 h post-treatment(P<0.05). Conclusion The improvement in pain in aged rats with hip fracture by fentanyl citrate may be associated with the reduction of SP expression in the spinal cord that inhibits glial cell activation and exhibits anti-inflammatory effects.