Background: We aimed to evaluate the effect of doxylamine, a first generation antihistamine, as a local analgesic agent by comparing its effect to bupivacaine. Methods: After blocking the cutaneous trunci muscle reflex (CTMR) by subcutaneous injection of doxylamine, we assessed doxylamine's cutaneous analgesic effect in rats. The dose-related effect and duration of doxylamine on infiltrative cutaneous analgesia were compared with that of bupivacaine. Results: We demonstrated that doxylamine, as well as the local anesthetic bupivacaine produced the cutaneous analgesic effects in a dose-related fashion. At the equipotent dose (50% effective doses (ED50)), the relative potency was bupivacaine (0.41 (0.36–0.48) mmol)> doxylamine (7.39 (6.91–7.91) mmol) (p < 0.01). On an equipotent basis (ED25, ED50 and ED75), subcutaneous doxylamine resulted in greater duration of action (p < 0.01) than bupivacaine at producing cutaneous analgesia. Conclusions: The result of this experiment indicated that doxylamine has the local anesthetic property less potent than bupivacaine, but its nociceptive block duration is longer than that of bupivacaine at an equianalgesic dose.
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