Diphenhydramine

Diphenhydramine is a first-generation H1 antihistamine with additional central sedative and antimuscarinic properties. Veterinary use centers on symptomatic relief of histamine-associated allergic signs in dogs and cats. Its effects on vestibular and emetic pathways also support empirical use for motion-associated nausea and vomiting, particularly in dogs.

Group

First-generation H1 antihistamine

Mechanism of Action

Diphenhydramine inhibits histamine H1-receptor signaling in peripheral tissues and the central nervous system. Peripheral blockade reduces histamine-mediated capillary leakage, swelling and stimulation of itch pathways. Central H1 and muscarinic receptor effects reduce signaling in vestibular pathways involved in motion sickness. These receptor effects also account for its sedative activity.

Dose Rates

Dog

Cat

Ferret

Guinea Pig

Rabbit

Bird

Horse

Cattle

Indications

• Symptomatic treatment of acute histamine-mediated cutaneous allergic reactions in dogs and cats, including urticaria and insect-bite-associated swelling.
• Adjunctive treatment of allergic pruritus in dogs and cats, with variable and often limited clinical benefit.
• Prevention or treatment of motion-associated nausea and vomiting in selected dogs.
• Adjunctive H1 blockade in veterinary patients with histamine-releasing mast-cell disease.

Contraindications

• Glaucoma, ileus or significant arrhythmias where antimuscarinic effects are undesirable

Interactions

• May oppose prokinetic parasympathomimetic drugs.

Adverse effects

• Sedation
• Dry mouth and reduced secretions
• Excitement at high doses, especially in cats

Preparations

Monitoring and Administration

Stop at least 7 days before canine intradermal allergy testing. Feline antiemetic efficacy is poor.

Large-animal source regimen is 0.5–1 mg/kg IM once as needed, without named species; not assigned to individual species.