Hydroxyzine
Hydroxyzine hydrochloride, Hydroxyzine pamoate
Also known as: Atarax · Vistaril · Masmoran · Ucerax · Hydroxyzine embonate · Hydroxyzine pamoate · Hydroxyzine HCl · Hydroxyzini HCl · Hydroxyzine hydrochloride
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Audited v13 dosing evidence
Structured calculator evidenceAntipruritic/antihistamine (extra-label)
- q8-12h
NA
Governing clinical context (2)
- ■ This medicine may be given with or without food. If your animal drools, vomits, or acts sick after receiving this medicine on an empty stomach, give with food or a small treat to see if this helps.
- Hydroxyzine oral products should be stored in tight, light-resistant containers at room temperature 20°C to 25°C (68°F-77°F). Hydroxyzine injection should be stored protected from light at room temperature 20°C to 25°C (68°F-77°F) with excursions permitted to 15°C to 30°C (59°F-86°F). Avoid freezing all liquid products.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
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 of action
Hydroxyzine acts as a competitive antagonist at H1-receptors on effector cells.
- Histamine Blockade: Competes with histamine for H1-receptor sites → 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 → binds to central H1 receptors → 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.
Safety & warnings
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)
Precautions
Use with caution in patients with prostatic hypertrophy, bladder neck obstruction, severe cardiac failure, angle-closure glaucoma, or pyeloduodenal obstruction due to its anticholinergic effects.
Pregnancy Warning: At doses substantially greater than those used therapeutically, hydroxyzine has been shown to be teratogenic in lab animals. Use during pregnancy (particularly during the first trimester) only when the benefits outweigh the risks (FDA Category C). It is unknown if hydroxyzine enters maternal milk, but its active metabolite cetirizine does.
Drug interactions
Additive anticholinergic effects may occur when used concomitantly.
Additive CNS depression may be seen.
Hydroxyzine may inhibit or reverse the vasopressor effects of epinephrine; use norepinephrine or metaraminol instead.
Additive CNS depression and sedation
Monitoring
- Efficacy (reduction in pruritus or clinical signs)
- Adverse effects (degree of sedation, excitability, or behavioral changes)
- Resolution of pruritus or allergic signs
- Adverse anticholinergic effects (e.g., monitoring urination)
Pharmacokinetics
Half-life
Absorption
Rapidly and well absorbed after oral administration.
Distribution
Readily crosses the blood-brain barrier. It is unknown if hydroxyzine enters maternal milk, but its active metabolite cetirizine has been detected in milk.
Metabolism
Apparently metabolized in the liver (to its active metabolite, cetirizine).
Elimination
Effects generally persist for 6-8 hours in dogs and up to 12 hours in cats.
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.
Available products
Formulations
- Oral Tablets
- Oral Syrup
- Injection
- Oral Capsules
- Oral Suspension
- 10 mg tablet
- 25 mg tablet
- 2 mg/ml syrup
Human-labeled
- Hydroxyzine HCl Oral Tablets: 10 mg, 25 mg, & 50 mg
- Hydroxyzine HCl Oral Syrup: 10 mg/5 mL
- Hydroxyzine HCl Injection: 25 mg/mL & 50 mg/mL
- Hydroxyzine Pamoate Oral Capsules: 25 mg, 50 mg, & 100 mg
- Hydroxyzine Pamoate Suspension: 25 mg/5 mL
- Atarax 10 mg tablets
- Atarax 25 mg tablets
- Ucerax 2 mg/ml syrup
Regulatory status
Classified as POM (Prescription Only Medicine).
Classified as POM (Prescription Only Medicine).
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Hydroxyzine oral products should be stored at room temperature in tight, light-resistant containers. Avoid freezing all liquid products.
Client information
- Drowsiness is common: Your pet may seem sleepy or lethargic, especially when starting the medication. This is normal, but contact your veterinarian if it seems excessive.
- Working Dogs: This medication can impede the performance and alertness of working, hunting, or service dogs.
- Consistency is key: Antihistamines often work best when given consistently to prevent allergic flare-ups rather than treating them once they are severe.
- Allergy Testing: If your pet is scheduled for intradermal skin allergy testing, you will need to stop this medication 3 to 7 days prior. Consult your veterinarian for exact timing.
- Do not double dose: If you miss a dose, give it when you remember, but never give two doses at once.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.