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.