Histamine 2 blocker potentiates the effects of histamine 1 blocker in suppressing histamine-induced wheal

Background : Histamine is responsible for the wheal and flare reaction in various allergic conditions. Classical antihistamines are the drugs which block the H₁ receptors and are widely used in various allergic conditions, whereas H₂ blockers are mainly used for acid peptic disease. Although H₁ receptor-mediated actions of histamine are primarily responsible for vasodilatation, vasopermeability, and itching, it has been observed that combined blocking of both H₁ and H₂ receptors may provide better relief. Aim: To compare the efficacy of levocetirizine (H₁ blocker) versus levocetirizine and ranitidine (H₂ blocker) in suppressing histamine-induced wheal. Methods: Fifteen volunteers were given a single dose of levocetirizine 5 mg on day 1 and a single dose of levocetirizine 5 mg with ranitidine 150 mg twice a day on day 7. A pretest was performed by intradermal histamine prick test. After administration of the drugs, the prick test was repeated at 1 hour, 2, 3, 6, and 24 hours, and the size of the wheal measured and statistically analyzed. Results: At 1 hour, there was no statistically significant difference in the wheal size between levocetirizine alone and the combination of levocetirizine and ranitidine. Levocetirizine with ranitidine resulted in statistically significant reduction of wheal size at 2, 3, 6, and 24 hours when compared with levocetirizine alone. Conclusion: H2 blocker potentiates the effects of an H1 blocker in suppressing histamine-induced wheal.