Hydroxyzine

Hydroxyzine is a first-generation piperazine antihistamine with sedative properties. It is used empirically for selected allergic and pruritic disorders in dogs, cats and other veterinary patients. Cetirizine is an active metabolite. Suppression of experimental histamine responses does not consistently translate into relief of chronic atopic itching in clinical patients.

Group

H1 antihistamine

Mechanism of Action

Hydroxyzine inhibits histamine H1-receptor signaling in peripheral tissues and the central nervous system. Peripheral effects reduce histamine-mediated vascular leakage and stimulation of itch pathways, while central H1 activity contributes to sedation. Additional antimuscarinic effects influence secretions and other autonomic functions. Conversion to cetirizine contributes to the overall antihistamine effect after administration.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Pruritus	2 mg/kg	PO, IM	Every 8–12 h
Pruritus	2.2 mg/kg	PO	Every 8 h

Horses

Use / stage	Dose	Route	Schedule	Instructions
Hypersensitivity and sedation	Reference only	PO		Hydroxyzine pamoate 500 mg per horse twice daily; efficacy has not been established. Pamoate must not be calculated using hydrochloride preparations.
Hypersensitivity	0.5–1 mg/kg	IM, PO	Every 12 h
Hydroxyzine pamoate hypersensitivity	Reference only	PO		0.67 mg/kg every 12 hours of the pamoate presentation; distinguish the salt/equivalent basis before selecting a product.

Cats

Use / stage	Dose	Route	Schedule	Instructions
Pruritus	1–2 mg/kg	PO	Every 8–12 h
Pruritus	5–10 mg/animal	PO	Every 8–12 h
Pruritus	2.2 mg/kg	PO	Every 8–12 h
Recurrent idiopathic lower urinary tract signs	5–10 mg/animal	PO	Every 12 h	Historical adjunct; assess underlying urinary disease.

Ferrets

Use / stage	Dose	Route	Schedule	Instructions
Pruritus	2 mg/kg	PO	Every 8 h

Birds

Use / stage	Dose	Route	Schedule	Instructions
Pruritus or feather self-trauma	2 mg/kg	PO	Every 8 h	Adjust to avoid excessive drowsiness.
Pruritus or feather self-trauma	2 mg/kg	PO	Every 12 h	Adjust to avoid excessive drowsiness.

Indications

Adjunctive symptomatic treatment of allergic pruritus in dogs and cats, with variable and often limited clinical response.
Symptomatic management of selected histamine-mediated hypersensitivity reactions, including urticaria.
Empirical adjunctive management of selected allergic rhinitis or airway-associated allergic signs, with unestablished clinical efficacy.

Contraindications

Discontinue at least 7 days before allergen-specific skin testing in dogs.
Use cautiously with urinary outflow obstruction, prostatic enlargement, severe heart failure, angle-closure glaucoma or pyloroduodenal obstruction.

Interactions

Other sedating drugs may increase central depression.
Other antimuscarinics increase adverse effects. Hydroxyzine may blunt or reverse the pressor response to epinephrine.

Adverse effects

Sedation, dry mouth and reduced gastrointestinal secretions

Toxicity

Excess exposure can intensify sedation and anticholinergic effects; provide symptom-directed care.

Preparations

Preparation	Form	Strength	Route
Hydroxyzine 10 mg tablet	Tablet	10 mg / tablet	PO
Hydroxyzine 25 mg tablet	Tablet	25 mg / tablet	PO
Hydroxyzine 50 mg tablet	Tablet	50 mg / tablet	PO
Hydroxyzine hydrochloride 2 mg/mL syrup	Oral Liquid	2 mg / mL	PO
Hydroxyzine 25 mg/mL injection	Injection	25 mg / mL	IM
Hydroxyzine pamoate 25 mg capsule	Capsule	25 mg / capsule	PO
Hydroxyzine pamoate 50 mg capsule	Capsule	50 mg / capsule	PO
Hydroxyzine hydrochloride 50 mg/mL injection	Injection	50 mg / mL	IM

Monitoring and Administration

Assess pruritus response rather than assuming histamine suppression predicts clinical benefit.

Hydrochloride and pamoate are distinct salts; do not exchange strengths without a stated equivalent basis.

Protect from light. A particular compounded syrup was stable for 14 days.

Bird drinking-water alternative: 1.5-2 mg per 4 US fluid ounces daily; actual intake determines exposure. Avoid freezing liquids. Some pamoate products state strength as hydrochloride equivalents; do not apply that conversion to every pamoate product.