Intraoperative magnesium sulphate as an analgesic adjunct in a multimodal, opioid-sparing protocol for canine ovariohysterectomy: a prospective, randomized, controlled, double-blind study

Introduction Magnesium acts as a non-competitive NMDA receptor antagonist and may attenuate central sensitization. Evidence regarding its analgesic- or anaesthetic-sparing effects in dogs remains inconclusive. This prospective, randomized, blinded, controlled clinical study evaluated the effect of intraoperative magnesium sulphate on postoperative pain in dogs undergoing ovariohysterectomy (OVH). Methods Twenty-eight bitches were randomly allocated to receive magnesium sulphate ( n = 14; 50 mg kg −1 IV dose followed by 20 mg kg −1 h −1 ) or saline ( n = 14). An a priori power analysis ( α = 0.05, power = 0.8) determined that 28 dogs were required to detect a 2-point difference in the short-form Glasgow Composite Measure Pain Scale (CMPS-SF) scores (SD 1.8; Cohen’s d = 1.11). Anaesthesia was induced with propofol and maintained with isoflurane. Postoperative pain (primary outcome) was assessed using the CMPS-SF for 48 h. Continuous variables were analyzed using independent-samples t -tests or repeated-measures ANOVA with Bonferroni correction; categorical variables were compared using Chi-square or Fisher’s exact test. Significance was set at p < 0.05. Results All dogs completed the study ( n = 28). Mean CMPS-SF scores at 1 h post-extubation were significantly lower in the Magnesium group compared with the Control group (3.9 ± 1.5 vs. 5.8 ± 1.9; p = 0.012). Pain scores varied over time ( F = 72.368, p < 0.01), with a significant time x treatment interaction ( F = 2.851, p = 0.004). Intraoperative fentanyl requirements (0.3 ± 0.6 vs. 0.6 ± 0.8 μg kg −1 ; p = 0.275) and postoperative rescue analgesia rates (21.4% vs. 35.7%; p = 0.673) did not differ between groups. The propofol induction dose was significantly lower in the Magnesium group (2.8 ± 0.5 vs. 3.7 ± 0.4 mg kg −1 ; p < 0.01), whereas end-tidal isoflurane concentrations were similar ( p = 0.112). Extubation time and time to first head movement were prolonged in the Magnesium group ( p < 0.01). Postoperative serum magnesium concentrations were significantly higher in the Magnesium group ( p < 0.01), while adverse effects did not differ between groups (35.7% vs. 21.4%; p = 0.683). Conclusion Intraoperative magnesium administration in dogs undergoing OVH, within a multimodal protocol, improved early postoperative analgesia and reduced propofol requirements without an opioid- or isoflurane-sparing effect. The dosing regimen was well tolerated, although recovery duration was prolonged. Clinical impact may be more evident in more invasive procedures or opioid-free approaches.