Co-Administration of Ondansetron Decreases the Analgesic Efficacy of Tramadol in Humans

Tramadol, a central analgesic acting on serotonin neurotransmission, is often co-used with ondansetron, a 5-HT(3) antagonist, for the management of postoperative pain to decrease nausea and vomiting. The aim of the study is to test the hypothesis that this drug combination raises tramadol requirement by patient-controlled analgesia (PCA). Forty patients undergoing hernioplasty or thyroidectomy were enrolled in a randomized, controlled study and allocated to receive ondansetron 4 mg i.v. (n = 20) or saline (n = 20). At 0, 1, 2, 4 and 24 h of PCA, tramadol consumption was evaluated. Tramadol consumption (mg × kg(-1) × h(-1)) was higher in the ondansetron group at the 2-hour time point compared to the control group (0.24 ± 0.1 vs. 0.17 ± 0.16; p = 0.01). Our study suggests that ondansetron acutely reduces the analgesic efficacy of tramadol.