Second-Generation Antihistamines

Cetirizine

Piperazine-derivative antihistamine

Therapeutic dose:

• Dogs: 1–4 mg/kg, PO, q 24 h

• Cats: 5 mg/cat, PO, q 12–24 h

Toxic dose:

• Unknown, but well tolerated by dogs and cats. Minimum lethal dose is 237 mg/kg in mice, and 562 mg/kg in rats.

• Peak plasma concentrations in 4–7 h

• Highly protein bound

• 80% eliminated in urine, primarily as unchanged drug

Within 5 h of ingestion

Fexofenadine

Butyrophenone-derivative antihistamine

Therapeutic dose:

• Dogs: 2–5 mg/kg, PO, q 12–24 h

• Cats: 10–15 mg/cat, PO, q 12–24 h

Toxic dose:

• No evidence of toxicosis observed in dogs receiving oral doses up to 2 g/kg.

• Often found in combination with pseudoephedrine, which can result in more serious clinical signs.

• Peak plasma concentrations in 2–3 h

• Highly protein bound

• 5% of oral dose is metabolized

• Primarily eliminated in feces

Within 60 min of ingestion

Loratadine

Tricyclic, long-acting histamine with selective peripheral histamine H₁-receptor antagonist activity

Therapeutic dose:

• Dogs: 0.25–1.1 mg/kg, PO, q 24 h

• Cats: 0.5 mg/kg, PO, q 24 h

Toxic dose:

• Mild clinical signs can occur at < 10 mg/kg. No deaths have been reported at oral dosages up to 5 g/kg in rats and mice.

• Often found in combination with pseudoephedrine, which can result in more serious clinical signs.

• Peak plasma concentrations in 1 h

• Loratadine highly protein bound; metabolite less protein bound

• Extensively metabolized in liver to active metabolite (desloratadine)

• Eliminated as metabolites in urine and feces

Within 1–4 h of ingestion