Chemical Stability of Diphenhydramine in "Magic Mouthwash" Stored at Room and Refrigerated Temperatures for 90-Days.

Objective: This study aimed to investigate the chemical stability of diphenhydramine in a pediatric "Magic Mouthwash" preparation, specifically a 1:1 mixture of aluminum hydroxide/magnesium hydroxide/simethicone (Mylanta comparable product) and liquid diphenhydramine over 90 days under different storage conditions.

Methods: A high-performance liquid chromatography-ultraviolet method was developed for quantifying diphenhydramine in the mouthwash. A total of 10 bottles of mouthwash were prepared, with half stored in the refrigerator and half kept at room temperature. The method was applied to analyze the stability of diphenhydramine in the mouthwash preparations, with 5-mL aliquots removed from each bottle at 0, 1, 7, 14, 30, 60, and 90 days. Stability was defined as maintaining 90-110% of the initial concentration.

Results: Both storage conditions (room temperature: 19.3 ± 0.8°C; refrigeration: 3.01 ± 0.3°C) maintained stable temperatures. The pH remained stable (room temperature: 8.34 ± 0.4; refrigeration: 8.38 ± 0.4). Diphenhydramine concentrations stayed within the 90-110% range for the entire study duration under both conditions. No statistically significant differences in diphenhydramine concentration were observed between storage conditions or over time.

Conclusion: The pediatric "Magic Mouthwash" demonstrated stable pH and diphenhydramine potency over 90 days, regardless of whether it was stored at room temperature or refrigerated. This supports the feasibility of bulk preparation and extended storage of this formulation, providing a safe and effective alternative to lidocaine-containing mouthwash for pediatric patients.