<p>Diphenhydramine is quickly absorbed after oral administration with maximum activity occurring in approximately one hour <sup class="text-reference-group"><a class="reference-popover-link" data-content="Paton DM, Webster DR: Clinical pharmacokinetics of H1-receptor antagonists (the antihistamines). Clin Pharmacokinet. 1985 Nov-Dec;10(6):477-97. (PubMed ID 2866055)" href="#reference-A644">6</a>,<a class="reference-popover-link" data-content="Electronic Medicines Compendium: Nytol One-A-Night (diphenhydramine hydrochloride) Monograph" href="#reference-L5287">13</a></sup>. The oral bioavailability of diphenhydramine has been documented in the range of 40% to 60%, and peak plasma concentration occurs about 2 to 3 hours after administration <sup class="text-reference-group"><a class="reference-popover-link" data-content="Paton DM, Webster DR: Clinical pharmacokinetics of H1-receptor antagonists (the antihistamines). Clin Pharmacokinet. 1985 Nov-Dec;10(6):477-97. (PubMed ID 2866055)" href="#reference-A644">6</a>,<a class="reference-popover-link" data-content="Electronic Medicines Compendium: Nytol One-A-Night (diphenhydramine hydrochloride) Monograph" href="#reference-L5287">13</a></sup>.</p>