Antihistamines appear to compete with histamine for cell receptor sites on effector cells.
Capsules and Elixir A single oral dose of diphenhydramine HCl is quickly absorbed with maximum activity occurring in approximately one hour. The duration of activity following an average dose of diphenhydramine HCl is from four to six 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.