It occurs as a white, crystalline powder and is freely soluble in water and alcohol and has a molecular weight of 291.82. Each diphenhydramine HCl capsule contains 25 mg or 50 mg diphenhydramine HCl for oral administration. Each Benadryl 25-mg capsule also contains lactose and magnesium stearate. Each Benadryl 50-mg capsule also contains confectioner's sugar and talc. The solutions for parenteral use have been adjusted to a pH between 5.0 and 6.0 either with sodium hydroxide or hydrochloric acid.
Antihistamines appear to compete with histamine for cell receptor sites on effector cells.
Capsules and Elixer A single oral dose of diphenhydramine HCl is quickly absorbed with maximum activity occurring in approximately 1 hour. The duration of activity following an average dose of diphenhydramine HCl is from 4 to 6 hours. Injection Diphenhydramine HCl in the injectable form has a rapid onset of action. Detailed information on the pharmacokinetics of diphenhydramine HCl injection is not available. Injection Diphenhydramine HCl in the injectable form should only be used when the oral forms are impractical.