Objectives: To study the drug interaction between gabapentin and morphine in surgical patients under general anesthesia.

Materials and Methods: Two hundred sixty patients undergoing surgery under general anesthesia were randomized into two groups, A with 130 patients receiving gabapentin 2.0 to 3.5 mg/kg orally for premedication add-on, and B with 130 patients getting morphine 0.1 to 0.2 mg/kg intravenously. After surgery, a co-researcher assessed patients using Ramsay sedation scale (RSS) and pain numeric rating scale (NRS) at 2, 4, 8, 12, 16, and 24-hour intervals.

Results: Two hundred thirty-two patients were included the present study without procedural adverse events. There were 120 and 112 patients in gabapentin and morphine group, respectively. The administration dosage of gabapentin and morphine between the two groups showed statistically significant differences (p=0.031). During the emergence, the RSS on the sedation, agitation, drowsiness, and pain scores of gabapentin (1.8±0.4) and morphine (1.7±0.5) appeared statistically significant differences (p=0.032); however, the RSS on that in the post-anesthetic care unit (PACU) were 2.0±0.1and 2.0±0.2, respectively, which showed insignificant differences (p-value 0.283).

Conclusion: A small, single oral dose of gabapentin as premedication showed a synergistic effect on intraoperative morphine administration. However, this additive effect was not long lasting through the PACU and might not be suitable for an extended surgery.