BackgroundIntrathecal morphine (ITM) injection is an effective postoperative analgesic strategy in open or laparoscopic donor hepatectomy; however, the optimal dose has not been determined. In this trial, we compared the post‐operative analgesic effects of two doses (300 vs. 400 μg) of ITM injections.MethodsIn this prospective randomized non‐inferiority trial, 56 donors were divided into either the 300 μg or 400 μg ITM group (n = 28, each). The primary outcome was the resting pain score at 24 h postoperatively. Pain scores, cumulative opioid consumption, and side effects (postoperative nausea and vomiting [PONV]) were compared up to 48 h postoperatively.ResultsFifty‐five donors participated in the entire study. The mean resting pain scores at 24 h after surgery were 1.7 ± 1.6 and 1.7 ± 1.1 in the ITM 300 and ITM 400 groups, respectively (mean difference, 0 [95% CI, −.8 to .7], p = .978). The upper limit of the 95% CI was lower than the prespecified non‐inferiority margin (δ = 1), indicating that non‐inferiority had been established. The incidence of PONV was lower in the ITM 300 group than in the ITM 400 group at 18 (p = .035) and 24 h postoperatively (p = .015). There were no significant differences in the resting and coughing pain scores and cumulative opioid consumption at any time point.ConclusionFor laparoscopic donor hepatectomy, preoperative ITM 300 μg exhibited non‐inferior postoperative analgesic effects compared to ITM 400 μg, with a lower incidence of PONV.