BackgroundDexmedetomidine is frequently combined with ropivacaine in ultrasound-guided transversus abdominis plane (TAP) block for analgesia after cesarean section (CS), but is associated with adverse effects. The ideal dose of dexmedetomidine as local anesthetic adjuvant has not been determined. This study aimed to explore the dose of perineural dexmedetomidine for ensuring analgesic efficacy while minimizing adverse effects.MethodsPatients undergoing CS under combined spinal-epidural anesthesia were randomly allocated to five groups: DR (0.00 μg/kg dexmedetomidine), DR1 (0.25 μg/kg), DR2 (0.50 μg/kg), DR3 (0.75 μg/kg), and DR4 (1.00 μg/kg), all combined with ropivacaine for TAP block. The co-primary outcomes were time to first rescue analgesia and total consumption of sufentanil at 48 h. The secondary outcomes included visual analogue scale (VAS) scores; Ramsay Sedation Scale scores; patient satisfaction scores; time to first ambulation; length of hospital stay; and postoperative complications. Success was defined as achieving statistical significance for both primary endpoints.ResultsAmong 150 enrolled patients, 12 were excluded for protocol violations, leaving 138 participants in the final analysis. With increasing doses of dexmedetomidine, time to first rescue analgesia was significantly prolonged (from 5.42 ± 2.29 h to 11.26 ± 2.48 h; P < 0.001), and total consumption of sufentanil at 48 h was markedly reduced (from 86.62 ± 16.58 µg to 47.44 ± 15.41 µg; P < 0.001), indicating a dose-dependent effect. In pairwise comparisons, groups DR3 and DR4 were both superior to lower-dose groups in prolonging time to first rescue analgesia, reducing total consumption of sufentanil, and shortening time to first ambulation, with no significant difference between DR3 and DR4. With increasing dexmedetomidine doses, the incidence of sinus bradycardia (P < 0.001) increased, while the incidence of postoperative nausea and vomiting (P < 0.001) decreased. No significant differences were observed among the five groups for VAS pain scores (group × time interaction effects), length of hospital stay, incidence of hypotension, and patient satisfaction scores.ConclusionDexmedetomidine as an adjuvant to ropivacaine in ultrasound-guided TAP block for analgesia after CS, demonstrated a dose-dependent effect. 0.75 μg/kg dexmedetomidine appeared to provide the most favorable balance between analgesic efficacy and safety.Clinical Trial Registrationhttps://www.chictr.org.cn, identifier ChiCTR2300068207.