Study objective: We compared the efficacies of cimetidine (an H2-receptor antagonist) and diphenhydramine (an H1-receptor antagonist) alone and in combination for alleviation of symptoms of acute allergic reactions. STUDY DESIGN AND INTERVENTIONS: In this prospective, randomized, double-blind study, patients and examiners assessed the severity of symptoms and signs of acute allergic reactions using a visual-analog scale before treatment and 30 minutes after treatment with 300 mg IV cimetidine and placebo, 50 mg IV diphenhydramine and placebo, or diphenhydramine plus cimetidine.
Setting and participants: Thirty-nine patients with acute allergic reactions presenting to two emergency departments of teaching hospitals.
Results: Of the 35 patients with pruritus, all 12 receiving diphenhydramine placebo had clinically significant relief compared with six of ten (60%) receiving cimetidine plus placebo (P = .03). Twelve of 13 (92%) receiving diphenhydramine plus cimetidine had relief, which was not a significant difference from the single drugs. Comparison of mean differences in pretreatment and post-treatment symptom scores (relief scores) among groups of patients with pruritus detected significantly more relief in the group receiving diphenhydramine plus placebo (80.3 +/- 7.4) than in those receiving cimetidine plus placebo (48.8 +/- 13.4) (P = .022). Of the 33 patients with urticaria, five of 11 (46%) receiving diphenhydramine plus placebo had clinically significant relief compared with eight of ten (80%) receiving cimetidine plus placebo (P = .18). Eleven of 12 patients (92%) receiving diphenhydramine plus cimetidine had relief, which is a significant difference from those receiving diphenhydramine plus placebo (P = .027). Comparison of mean relief scores in patients with urticaria detected significantly more relief in the group receiving diphenhydramine plus cimetidine (55.3 +/- 6.5) than in the group receiving diphenhydramine plus placebo (30.7 +/- 6.1) (P = .006).
Conclusion: For treatment of pruritus from acute allergic reactions, diphenhydramine is more effective than cimetidine, and the combination offers no additional benefit. For treatment of acute urticaria, the combination of cimetidine and diphenhydramine is more effective than diphenhydramine alone.