<p>Diphenhydramine is a first-generation histamine H1 receptor antagonist (H1 antihistamine) that is widely available as a non-prescription, over-the-counter (OTC) medication. As an OTC medication, diphenhydramine is typically formulated as tablets and creams indicated for use in treating sneezing, runny nose, itchy/watery eyes, itching of nose or throat, insomnia, pruritis, urticaria, insect bites/stings, allergic rashes, and nausea <sup class="text-reference-group"><a class="reference-popover-link" data-content="StatPearls [Internet]: Diphenhydramine" href="#reference-L5263">9</a>,<a class="reference-popover-link" data-content="Electronic Medicines Compendium: Histergan (diphenhydramine hydrochloride) Tablets Patient Information" href="#reference-L5266">10</a>,<a class="reference-popover-link" data-content="Electronic Medicines Compendium: Histergan (diphenhydramine hydrochloride) Cream Monograph" href="#reference-L5269">11</a>,<a class="reference-popover-link" data-content="DIPHENHYDRAMINE- diphenhydramine hcl tablet, coated DailyMed FDA Label" href="#reference-F3379">17</a>,<a class="reference-popover-link" data-content="Flake ZA, Scalley RD, Bailey AG: Practical selection of antiemetics. Am Fam Physician. 2004 Mar 1;69(5):1169-74. (PubMed ID 15023018)" href="#reference-A174541">5</a></sup>.</p><p>Additionally, when the use of oral diphenhydramine is impractical, there are also prescription-only formulations such as diphenhydramine injection products that are effective in adults and pediatric patients (other than premature infants and neonates) for:
i) the amelioration of allergic reactions to blood or plasma, in anaphylaxis as an adjunct to epinephrine and other standard measures after acute allergic reaction symptoms have been controlled, and for other uncomplicated allergic conditions of the immediate type when oral therapy is impossible or contraindicated <sup class="text-reference-group"><a class="reference-popover-link" data-content="Diphenhydramine Hydrochloride Injection, USP FDA Label" href="#reference-F3352">16</a></sup>;
ii) the active treatment of motion sickness <sup class="text-reference-group"><a class="reference-popover-link" data-content="Diphenhydramine Hydrochloride Injection, USP FDA Label" href="#reference-F3352">16</a></sup>; and
iii) use in parkinsonism when oral therapy is impossible or contraindicated, as follows: parkinsonism in the elderly who are unable to tolerate more potent agents; mild cases of parkinsonism in other age groups, and in other cases of parkinsonism in combination with centrally acting anticholinergic agents <sup class="text-reference-group"><a class="reference-popover-link" data-content="Diphenhydramine Hydrochloride Injection, USP FDA Label" href="#reference-F3352">16</a></sup>.</p>