Objectives Patients can benefit from the coadministration of several medications because of the shorter infusion time and more rapid administration. The use of extemporaneously prepared admixtures of dexamethasone sodium phosphate (DSP) and 5-HT3 receptor antagonists (5-HT3RAs) must be supported by sufficient documentation of their compatibility. The objective of this study was to comprehensively investigate the compatibility of DSP with 5-HT3RAs in infusion solutions. Methods Admixtures of DSP with six different 5-HT3RAs (ondansetron hydrochloride, tropisetron hydrochloride, dolasetron mesylate, azasetron hydrochloride, palonosetron hydrochloride and ramosetron hydrochloride) were prepared in non-polyvinyl chloride (non-PVC) infusion bags filled with 5% glucose or 0.9% NaCl. Bags were stored at ambient temperature (25 +/- 2 degrees C) without protection from light. Samples were taken immediately after preparation (0 hour) and at predetermined intervals (12, 24 and 48 hours after preparation). Particulate matter of admixtures was inspected visually and particles were counted with a particle counter. The pH of each sample was also determined. Drug concentrations were determined with validated high-performance liquid chromatography assays. Results No visible haze or particulate formation, colour change or gas evolution and no notable changes in pH were observed, and particulate matter was acceptable up to 48 hours. All preparations maintained more than 90.0% of the initial concentration over the study period. Conclusions All the admixtures of DSP and the 5-HT3RAs studied were compatible and stable for at least 48 hours in a 5% glucose injection or a 0.9% NaCl injection stored in non-PVC infusion bags under ambient conditions.