Cetirizine (Zyrtec) dosing in dogs is commonly:
1 mg/kg PO every 12–24 hours, with a practical usual dose of 10–20 mg per dog. Merck also lists 0.5–1 mg/kg q12h for canine atopic dermatitis and 1–4 mg/kg q24h for urticaria. (Merck Veterinary Manual)
There’s a wide “therapeutic index” meaning a little, or a LOT is safe. And people still use Zyrtec because they’re taking it, and they have it around. Most people prefer Allegra because it works better at once-a-day dosing.
You can use Ceterizine / Zyrtec once a day if you want.
For routine itching/allergy use, a simple clinic-style chart would be roughly:
| Dog weight | Cetirizine |
|---|---|
| 5–10 lb | 2.5–5 mg |
| 11–20 lb | 5–10 mg |
| 21–40 lb | 10 mg |
| 41–70 lb | 10–20 mg |
| >70 lb | 20 mg |
Important: use plain cetirizine only. Avoid Zyrtec-D, because the pseudoephedrine component can be dangerous for dogs.
If you tell me the dog’s weight and what you’re treating—itching, hives, vaccine reaction, etc.— I can give you the dose I’d use in that situation.
