Association between preoperative metformin exposure and postoperative nausea and vomiting in patients undergoing general anaesthesia: a protocol for a prospective observational cohort study in a Chinese tertiary hospital

Introduction Postoperative nausea and vomiting (PONV) is a common and distressing complication following surgery, persisting despite advances in prophylactic regimens. Growth differentiation factor-15 (GDF-15) has been identified as a biomarker inversely associated with PONV risk and severity.

As metformin is known to elevate circulating GDF-15 levels, we hypothesise that preoperative metformin use may be associated with a lower incidence of PONV. This study aims to evaluate the association between a history of preoperative metformin administration and the occurrence of PONV in adults undergoing general anaesthesia.

Methods and analysis This is a single-centre, prospective, observational cohort study. We plan to enrol 909 adult patients scheduled for surgery under general anaesthesia with endotracheal intubation from December 2025 to December 2028.

Participants will be divided into two groups based on their preoperative metformin exposure: an exposed group (n=303) with a documented history of metformin use and a non-exposed group (n=606) without such history, using a 1:2 ratio. The metformin regimen (choice of agent and daily dosage) will be determined by the attending physician as part of routine clinical care, independent of this study.