Background and Objectives. The aim of this study was to determine if pre-emptive systemic fentanyl administration would decrease the amount of formalin pain in mice.

Methods. Under halothane anesthesia, formalin was injected in the hindpaw of mice. The animals were randomly assigned to receive no systemic fentanyl (group 1; n = 20); subcutaneous fentanyl, 50 μg/kg 5 minutes prior to formalin injection (group 2; n = 20); or subcutaneous fentanyl, 50 μg/kg 5 minutes after formalin injection (group 3; n = 20). After recovery from anesthesia, the formalin-induced paw-licking response was observed for 1 hour. The time to first paw lick, the number of paw licks, and the duration of paw licking were compared among the groups with an analysis of variance.

Results. The fentanyl-treated mice in groups 2 and 3 had a significantly longer time to first paw lick, had a lower number of paw licks, and had a shorter duration of paw licking compared to the control mice in group 1. There was no significant difference in formalin pain behavior between the mice receiving fentanyl before or after formalin injection.

Conclusions. Pre-emptive administration of subcutaneous fentanyl does not decrease the severity of acute pain in mice.