DiphenhydramineDose, contraindications and adverse effects for Cats, Cattle, Dogs, Horses and Poultry.

First-generation H1 antihistamine; also has sedative and mild anticholinergic effects. Diphenhydramine is used primarily for its antihistaminic effects. In dogs and cats, diphenhydramine may be used to manage allergic and pruritic conditions, including urticaria, angioedema, and insect bite hypersensitivity.

Diphenhydramine has antihistaminergic (H1), antiemetic, anticholinergic, CNS depressant, and local anesthetic effects. Diphenhydr- amine did not suppress wheal response after intradermal histamine administration.

Diphenhydramine is contraindicated in patients that are hypersensitive to it or other antihistamines in its class. Because of their anticholinergic activity, antihistamines should be used with caution in patients with angle-closure glaucoma, prostatic hypertrophy, pyloroduodenal or bladder neck obstruction, and asthma or chronic obstructive pulmonary disease if mucosal secretions are a problem.

The most common adverse effects are CNS depression (eg, lethargy, sedation) and anticholinergic effects (eg, constipation, dry mouth, urinary retention). The sedative effects of antihistamines may be dose-dependent and diminish with chronic administration. GI effects (eg, diarrhea, vomiting, anorexia) are also possible. Oral diphenhydramine had no effect on tear production or intraocular pressure in healthy dogs.

■ Clinical efficacy (trial periods to determine an individual antihistamine’s efficacy are usually 1 to 2 weeks long) ■ Adverse effects (eg, lethargy, sedation, constipation, urinary retention, vomiting, diarrhea, anorexia) Diphenhydramine 411