Effect of H1 blockers alone and in combination with morphine to produce antinociception in mice.

Antinociception was assessed in male CD-1 mice by a modification of Haffner's tail-clamp procedure. The H1 blockers, including an ethylenediamine (pyrilamine), an ethanolamine (diphenhydramine), a phenothiazine (methdilazine), a piperazine (cyclizine) and an alkylamine (chlorpheniramine), all produced antinociception when given alone to mice and also caused potentiation when combined with morphine.