Interactions of the H1 antihistamines.

An interaction is taken to be the situation in which the administration of a drug or substance induces changes in the pharmacokinetics of another simultaneously administered drug – either increasing or decreasing the plasma concentration of the latter, and thus giving rise to the possibility of adverse reactions [1]. According to the primarily responsible pharmacological mechanism, H 1 antihistamine interactions are fundamentally of a pharmacokinetic nature: the triggering drug or substance induces changes in the absorption and/ or metabolism of the H 1 antihistamine [2]. Interactions of a pharmacodynamic nature, i.e., corresponding to those situations in which the actions of the drug or substance upon its target tissues induce modifi cations in the actions of another drug, have not been reported to date. The interactions described to date between the H 1 antihistamines and other drugs or substances fundamentally take place via three different routes: the P450 cytochrome system; P glycoprotein (PgP); and the members of the organic anion transport polypeptide (OATP) family.