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Please confirm that an ethics committee approval has been applied for or granted: Yes: I’m uploading the Ethics Committee Approval as a PDF file with this abstract submissionApplication for ESRA Abstract PrizesBackground and Aims Myofascial pain syndrome (MPS) is a common source of chronic musculoskeletal pain characterized by regional pain within the muscle, fascia or surrounding soft tissue. This syndrome is often managed through trigger point injections with local anesthetics. Ondansetron, a selective 5-HT3 antagonist primarily used as an antiemetic, has been suggested to exhibit local antinociceptive effects. The aim of this study was to compare the analgesic effectiveness of ropivacaine alone versus ropivacaine combined with ondansetron in patients with MPSMethods Forty people with a clinical diagnosis of MPS involving trigger points in the lumbar paraspinal muscles, quadratus lumborum and gluteal region were randomized into Group A, receiving injections of ropivacaine 0.2% alone and group B, receiving injections of ropivacaine 0,2% with ondansetron. Each trigger point was injected with 3 ml of the corresponding solution, with solutions for both groups visually identical. Pain intensity was assessed using the Numeric Rating Scale (NRS) before trigger point injection, at 72 hours, one week and two weeks post-injectionResults Both groups demonstrated reduction in pain scores following treatment. However, Group A exhibited significantly greater analgesic improvement at all follow-up time points, as measured by NRS scores, compared to Group B (p<0.001 at 72 hours and one week post injection and p=0.003 at two weeks post injection)Conclusions The addition of ondansetron to ropivacaine may antogonize its analgesic efficacy. This effect could be explained by a pharmacodynamic interaction, as ondansetron may modulate nociceptive neurotransmission through 5-HT 3 receptor antagonism and interfere with sodium channel-mediated local anesthetic action.