In dermographic urticaria H2 receptor antagonists have a small but therapeutically irrelevant additional effect compared with H1 antagonists alone

Two studies of the additional effect of an H₂ receptor antagonist when given in combination with an H₁ antagonist were undertaken in dermographic urticaria. Using a randomized, double-blind, crossover design in 19 patients, a combination of cetirizine (10 mg at night) and ranitidine (150 mg twice daily) was compared with a combination of cetirizine (10 mg at night) and placebo. The addition of ranitidine did not produce any significant difference in linear analogue scores for weal, Itch or sleep disturbance. There was a significant depression of the frictional force/wealing response curve with an increase in wealing threshold (P<0.0001) following the addition of H₂ blockade. The wealing threshold was 54.7 ± 4.4 (mean ± SEM) g/mm² for the H₁ antagonist alone, and 73.2 ± 5.7 for the combination of H₁ and H₂ antagonists. In a second similar study involving nine different patients, comparing terfenadine (120 mg twice daily) with a combination of terfenadine and ranitidine (150 mg twice daily), the weal threshold was 59.8 ± 6.6 for the H₁ antagonist alone, and 73.0 ± 6.4 for the combination of H₁ and H₂ antagonists. Thus, in dermographic urticaria, adding an H₂ antagonist to treatment with a potent H₁ antagonist gives a small, significant reduction in wealing response, but no symptomatic benefit. We conclude that involvement of the H₂ receptor in this urticarial disease is minimal, and does not justify the use of H₂ receptor antagonists.