Analgesic effects of antihistaminics.

The literature provides considerable evidence indicating that several, but not all antihistaminics, are indeed analgesic agents and some are analgesic adjuvants as well. Those for which effectiveness is reported includes diphenhydramine, hydroxyzine, orphenadrine, pyrilamine, phenyltoloxamine, promethazine, methdilazine, and tripelennamine. The proposed mechanisms of analgesic action of antihistaminics are reviewed and discussed. The literature suggests that more than one mechanism of action exists for them. There is considerable evidence suggesting that histaminergic and serotoninergic central pathways are involved in nociception and that antihistaminic drugs can modulate their responses (1). The evidence for a role for norepinephrine and dopamine and the effects of antihistaminics on them are less well established. Still other pathways have been proposed. A greater understanding of pain mechanisms will aid in elucidating the role of antihistaminics in analgesia.