Cetirizine Hydrochloride — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	0.5-1 mg/kg	q12h to q24h	Duration: As needed for symptom control; typically 5-14 days Notes: Start at lower end for mild pruritus. May be used long-term for chronic atopy.
Cat	PO	0.5-1 mg/kg	q12h to q24h	Duration: As needed; typically 5-14 days Notes: Cats may require higher end of dose. Use with caution in renal impairment.
Horse	PO	0.2-0.4 mg/kg	q12h	Duration: As needed for allergic episodes Notes: Limited studies; adjust based on response.
Rabbit	PO	0.5-1 mg/kg	q12h	Duration: As needed Notes: Off-label use; monitor for sedation.

Clinical Indications & Species Uses

General Indications

Symptomatic relief of allergic reactions
Reduction of histamine-mediated pruritus

Dog (Canine)

Pruritus associated with allergic dermatitis
Atopic dermatitis
Urticaria
Allergic rhinitis (adjunctive)
Histamine-mediated pruritus

Cat (Feline)

Pruritus associated with allergic dermatitis
Atopic dermatitis
Urticaria
Eosinophilic granuloma complex (adjunctive)
Feline miliary dermatitis

Horse (Equine)

Pruritus associated with insect bite hypersensitivity
Urticaria
Allergic dermatitis

Rabbit & Small Mammals

Pruritus associated with allergic dermatitis (off-label use)

Exotic & Other Species

Pruritus in small mammals (off-label use)

Pharmacology & Mechanism of Action

Drug Class: Antihistamine (Second Generation, H1 Receptor Antagonist) | Pharmacological Group: Piperazine derivative

Mechanism of Action: Cetirizine is a selective, long-acting peripheral H1 receptor antagonist. It competitively blocks histamine at the H1 receptor, preventing histamine-mediated effects such as vasodilation, increased vascular permeability, pruritus, and bronchoconstriction. It has minimal affinity for muscarinic, serotonin, and dopamine receptors, and does not penetrate the blood-brain barrier significantly, reducing central nervous system side effects.

Pharmacodynamics: Cetirizine inhibits the histamine-induced wheal and flare response, reduces pruritus, and decreases nasal and ocular symptoms of allergic reactions. It has anti-inflammatory properties, including inhibition of eosinophil chemotaxis and adhesion molecule expression. Onset of action is within 1-2 hours, with peak effect at 4-8 hours. Duration of action is approximately 24 hours.

⚡ Pharmacokinetics Summary

Absorption: Rapidly and well absorbed after oral administration. Peak plasma concentrations occur within 1-2 hours in dogs and cats. Food may slightly delay absorption but does not affect overall bioavailability.

Distribution: Widely distributed in tissues. Volume of distribution is approximately 0.5-0.8 L/kg in dogs. Protein binding is about 93% in humans, but in dogs it is approximately 70-80%. It does not cross the blood-brain barrier significantly.

Metabolism: Undergoes minimal hepatic metabolism. The primary metabolic pathway is oxidative dealkylation to form inactive metabolites. In dogs, metabolism is limited, and the drug is mostly excreted unchanged.

Excretion: Primarily excreted unchanged in urine (about 60-70% in dogs) and feces. Renal excretion involves both glomerular filtration and active tubular secretion. In cats, a significant portion is also excreted unchanged.

Half-Life: Dogs: approximately 10-12 hours; Cats: approximately 12-14 hours; Horses: approximately 4-6 hours; Humans: 8-9 hours.

Bioavailability: Oral bioavailability is high, approximately 70-85% in dogs and cats, with minimal first-pass metabolism.

Protein Binding: Approximately 70-80% in dogs, 60-70% in cats, and 93% in humans.

Available Formulations & Strengths

Oral Tablet 5 mg, 10 mg (PO)

Oral Solution 1 mg/mL (syrup) (PO)

Oral Disintegrating Tablet 10 mg (PO)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to cetirizine or other piperazine derivatives (e.g., hydroxyzine)
Severe renal impairment (creatinine clearance < 10 mL/min) unless dose adjusted
Use in animals with known history of urinary retention or bladder obstruction
Pregnancy and lactation unless benefits outweigh risks (limited safety data)

Warnings & Clinical Precautions:

Use with caution in animals with hepatic impairment, as metabolism may be reduced.
May cause sedation in some animals, especially at higher doses.
Use with caution in animals with seizure disorders; antihistamines may lower seizure threshold.
In cats, may cause paradoxical excitation.
Not recommended for use in animals with significant cardiovascular disease.
Avoid concurrent use with other CNS depressants.
In food animals, withdrawal times must be observed; extra-label use is prohibited in some countries.

Adverse Effects & Reactions

Common:

Sedation
Lethargy
Gastrointestinal upset (vomiting, diarrhea)
Dry mouth
Decreased appetite

Serious / Severe:

Paradoxical excitation (especially in cats)
Seizures (rare)
Hypotension
Urinary retention
Hepatotoxicity (rare)

Rare:

Blood dyscrasias
Allergic reactions (rash, anaphylaxis)
Cardiac arrhythmias

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
CNS depressants (e.g., barbiturates, opioids, benzodiazepines)	Additive sedative effects	Moderate
Anticholinergic drugs	Increased risk of urinary retention and dry mouth	Moderate
Ketoconazole or other CYP3A4 inhibitors	May increase cetirizine levels (though metabolism is minimal, potential for increased effects)	Mild
Ritonavir	May increase cetirizine exposure	Mild
Theophylline	May decrease clearance of cetirizine	Mild

Overdose & Toxicity Management

Signs of Toxicity:

Severe sedation
Paradoxical excitation
Tremors
Seizures
Respiratory depression
Hypotension
Coma

Emergency Treatment Protocol: Induce emesis if recent ingestion and animal is conscious. Administer activated charcoal to reduce absorption. Provide symptomatic and supportive care: IV fluids, respiratory support, anticonvulsants (e.g., diazepam) for seizures, and cardiovascular support. Monitor vital signs and ECG. There is no specific antidote.

Food Animal Withdrawal Times

Not approved for use in food animals. Extra-label use in food animals is prohibited in many jurisdictions. If used, consult regulatory authorities for withdrawal times.

Storage, Handling & Regulatory Information

Storage Temperature: Store at room temperature (20-25°C, 68-77°F)

Handling & Special Conditions: Protect from moisture. Keep in tightly closed container.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; used off-label in animals. Approved for human use.

Extra-Label (Off-Label) Use: In the US, extra-label use in animals is permitted under AMDUCA for non-food animals. For food animals, extra-label use is prohibited if there is no established withdrawal time and if the drug is not approved for that species.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Cetirizine is a second-generation antihistamine that is commonly used off-label in veterinary medicine for the management of pruritus and allergic conditions. It is generally well-tolerated, with sedation being the most common side effect, though less frequent than with first-generation antihistamines. In dogs and cats, it is often used as an alternative to hydroxyzine or diphenhydramine. Dosing is typically 0.5-1 mg/kg every 12-24 hours. Clinical response may take several days to become apparent. It is important to rule out other causes of pruritus (e.g., parasites, infections) before initiating therapy. In horses, cetirizine has been used for insect bite hypersensitivity, but evidence is limited. For food animals, use is not recommended due to lack of withdrawal data. Always monitor for adverse effects, especially in animals with renal or hepatic impairment. Patient response varies; some animals may require higher doses or more frequent administration. Combination therapy with omega-3 fatty acids or other antipruritic agents may be beneficial.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)