There are no published studies that report on fentanyl and ketamine compatibility in dextrose 5% solution as commonly practiced in hospital settings. This study assessed the compatibility of fentanyl and ketamine after their reconstitution in dextrose 5% under ambient temperature and humidity conditions in a hospital setting. Each sample of fentanyl and ketamine was prepared in triplicate by adding dextrose 5% to a prefilled syringe until final concentrations of 9.8 μg mL-1 for fentanyl and 192 μg mL-1 for ketamine were achieved. The solutions were stored in syringes at an ambient temperature ranging 26-28°C either under a mixture of daylight and ambient lighting. A sample was taken from the syringes at the following times: 0, 8, 24, 72, 120, and 168 hours. The samples from each solution were checked for physical changes, pH and their concentration assessed by high-performance liquid chromatography. The solutions were clear and no physical changes were seen. The pH of fentanyl and ketamine decreased dramatically after 72 hours. The concentrations of fentanyl remained 90-110% only for 24 hours, while ketamine remained 90-110% for 168 hours. Solutions of fentanyl and ketamine in dextrose 5% may be used in prefilled syringes only up to 24 hours and 72 hours after reconstitution, respectively.