2. H1 Antagonists • reversible competitive inhibitors • well absorbed from the GI tract; peak plasma concentrations are achieved in 2 to 3 hours, and effects usually last 4 to 6 hours • induce hepatic cytochrome P450 enzymes

3. H1 Antagonists • Primarily effective against cutaneous effects of anaphylaxis • Edema formation and itch - effectively suppressed. • Hypotension - less well antagonized (Gelfand, et al., 2004; Campbell and Harder, 1999) • Bronchoconstriction - reduced little, if at all.

4. H1 Antagonists: 1st generation • Diphenhydramine • significant antimuscarinic activity (drying and thickening of oral and other respiratory secretions) and have a pronounced tendency to induce sedation

5. H1 Antagonists: 2nd generation • Loratadine, Cetirizine • Less lipid-soluble and enter the central nervous system with difficulty or not at all • absorbed rapidly from the GI tract and metabolized in the liver to an active metabolite by the hepatic CYPs • excreted primarily into the urine as unmetabolized form

6. Recommendation • Adjuvant to epinephrine • no evidence from randomized controlled trials to support the use of H1-antihistamines in the emergency management of anaphylaxis • should be administered for all patients with anaphylaxis or generalized urticaria • continued for 2-3 days after treatment of the acute anaphylactic event