Hydroxyzine
Hydroxyzine is a first-generation piperazine-derivative antihistamine widely used in veterinary medicine. It is primarily prescribed for its **antipruritic** and **antihistaminic** properties, making it a staple in managing atopic dermatitis, flea allergy dermatitis, and other allergic conditions in dogs and cats. Because it readily crosses the blood-brain barrier, it also provides **sedative** and **tranquilizing** effects, which can be highly beneficial for anxious or intensely pruritic animals that are self-mutilating. In cats, it is occasionally utilized to manage frequently recurrent idiopathic lower urinary tract disease (FLUTD) due to its antispasmodic and mild anxiolytic properties.
Mechanism: Hydroxyzine acts as a competitive antagonist at **H1-receptors** on effector cells. * **Histamine Blockade**: Competes with histamine for H1-receptor sites โ prevents histamine-induced bronchoconstriction, vasodilation, and capillary permeability. *Note: It does not inhibit the release of histamine from mast cells, but antagonizes its effects.* * **CNS Effects**: Readily crosses the blood-brain barrier โ binds to central H1 receptors โ causes sedation and tranquilization. * **Additional Properties**: Exhibits anticholinergic, antispasmodic, local anesthetic, mild bronchodilative, and antiemetic activities. * **Metabolism Pearl**: In vivo, hydroxyzine is metabolized in the liver to its active metabolite, **cetirizine**, which contributes significantly to its prolonged antihistaminic effects.
Dosing by species
- For pruritus ยท 1-2 mg/kg or 5-10 mg/cat PO q8-12h ยท PO ยท q8-12h
- For pruritus ยท 5-10 mg (total dose) or 2.2 mg/kg PO q8-12h ยท PO ยท q8-12h
- For frequently recurrent idiopathic lower urinary tract disease ยท 5-10 mg (total dose) per cat PO q12h ยท PO ยท q12h
- General ยท 2 mg/kg PO 3 times daily ยท PO ยท q8h
- General ยท 0.5-1 mg/kg IM or PO twice daily ยท IM/PO ยท q12h ยท ARCI UCGFS Class 2 Drug
- General ยท 0.67 mg/kg PO twice daily ยท PO ยท q12h ยท Using the pamoate salt
- For pruritus associated with allergies, feather picking, or self-mutilation ยท 2 mg/kg q8h PO or 1.5-2 mg per 4 oz of drinking water daily ยท PO ยท q8h or daily ยท Adjust dose to minimize drowsiness and maximize effect
- For pruritus associated with allergies, feather picking, or self-mutilation ยท 2 mg/kg PO q12h ยท PO ยท q12h
- As an antipruritic/antihistamine ยท 2.2 mg/kg PO three times daily (q8h) ยท PO ยท q8h
Routes of administration
Contraindications
- Hypersensitivity to hydroxyzine or its metabolites (e.g., cetirizine)
- Known hypersensitivity to hydroxyzine or cetirizine
- Early pregnancy (teratogenic in some animal models)
Adverse effects
- Sedation (most common, usually mild and transient in dogs)
- Hyperexcitability (occasional paradoxical reaction)
- Fine rapid tremors (dogs)
- Whole body tremors (dogs)
- Seizures (rare in dogs)
- Polydipsia (cats)
- Depression (cats)
- Behavioral changes (cats)
- Sedation / Lethargy
- Dry mouth
- Urinary retention
- Gastrointestinal upset
- Paradoxical excitation (rare)
Drug interactions
- Anticholinergic agents ยท Additive anticholinergic effects may occur when used concomitantly. ยท moderate
- CNS Depressant Drugs (e.g., barbiturates, tranquilizers) ยท Additive CNS depression may be seen.
- Epinephrine ยท Hydroxyzine may inhibit or reverse the vasopressor effects of epinephrine; use norepinephrine or metaraminol instead.
- CNS Depressants (e.g., opioids, barbiturates, gabapentin) ยท Additive CNS depression and sedation ยท major
Monitoring
- Efficacy (reduction in pruritus or clinical signs)
- Adverse effects (degree of sedation, excitability, or behavioral changes)
- Resolution of pruritus or allergic signs
- Degree of sedation
- Adverse anticholinergic effects (e.g., monitoring urination)
Overdose
Overdoses would be expected to cause **increased sedation** and perhaps **hypotension**. At high doses, fine rapid tremors and rarely seizures have been reported. Common clinical signs of toxicity in dogs include: * Hyperthermia * Lethargy * Tachycardia * Trembling / Tremors * Ataxia * Somnolence **Treatment**: There are no specific antidotes available. Gut emptying protocols should be considered with large or unknown quantity overdoses. Supportive and symptomatic treatment is recommended if necessary.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.