Antihistamine Dose Calculator for Dogs and Cats
Safe starting doses for diphenhydramine, cetirizine, loratadine, chlorphenamine, fexofenadine and clemastine. Pick the drug, check the active ingredient and read both safety gates.
Call a vet first if
These are emergencies. Oral antihistamines are too slow to fix them and waiting can kill. Skip the calculator and go straight to a vet:
- Swollen face or muzzle with any trouble breathing, noisy breathing, or blue or pale gums
- Collapse, weakness, or vomiting after a sting, bite, vaccine, or new food or drug
- Hives spreading fast with restlessness and a racing heart together (early anaphylaxis)
- Your pet has heart disease, glaucoma, an enlarged prostate, a urinary blockage, or seizures, or is pregnant, very young, very old, or on other medications
Two safety gates before you see the dose
Tick both once you have genuinely checked. Not a formality.
The six at a glance
| Drug | Typical dog | Typical cat | How often | Drowsy? |
|---|---|---|---|---|
| Diphenhydramine (Benadryl) | 2 to 4 mg/kg | 2 to 4 mg/kg | Every 8 to 12 h | Yes |
| Cetirizine (Zyrtec) | 5 to 10 mg | 5 mg | Once daily | Minimal |
| Loratadine (Claritin) | 5 to 10 mg | Ask your vet | Once daily | No |
| Chlorphenamine (Piriton) | 0.2 to 0.4 mg/kg | 1 to 2 mg | Dog q8 to 12 h, cat q12 h | Yes |
| Fexofenadine (Allegra, Telfast) | 5 to 10 mg/kg | Ask your vet | Once daily | No |
| Clemastine (Tavegil, Tavist) | 0.05 to 0.1 mg/kg | 0.34 to 0.68 mg | Every 12 h | Yes |
The detail
Why the doses do not all scale the same way
Diphenhydramine, chlorphenamine, fexofenadine and clemastine are calculated from body weight. Cetirizine and loratadine use flat dose ranges. This calculator applies the appropriate method for each drug rather than forcing one formula across all six.
Drowsy or not
Diphenhydramine, chlorphenamine and clemastine are sedating. Cetirizine is minimally sedating, while loratadine and fexofenadine are considered non-drowsy.
The combination-product trap
Anything ending in “-D” may contain pseudoephedrine. “PM”, “Sinus”, “Plus” and “Cold & Flu” packs may contain paracetamol, ibuprofen, phenylephrine or additional sedatives. Only use a plain, single-ingredient antihistamine.
Fexofenadine and clemastine
Fexofenadine is a non-drowsy option used mainly in dogs; feline data are limited. Clemastine is sedating and has poor, variable oral absorption in dogs, so response can be unreliable.
Which one for a cat
Cetirizine and chlorphenamine are the better-established feline choices. Diphenhydramine can be used but is sedating. Loratadine and fexofenadine have limited feline data and should be confirmed with a vet first.
Signs you gave too much
- Heavy sedation, wobbliness or slowed breathing
- Agitation, fast heart rate, dilated pupils or tremors
- Seizures or breathing difficulty in severe overdose
Do antihistamines work for allergic itch?
They help only a minority of dogs with established atopic dermatitis and often work better for hives, insect bites and mild seasonal flare-ups. If itch remains significant, speak to your vet about more reliable prescription options.
Built by Dr Duncan Houston for ASK A VET™. Log which antihistamine, what dose, and how your pet reacted in one place, and reach a real vet when a number on a screen is not enough.
This calculator is general guidance, not a substitute for veterinary advice, diagnosis or treatment. These medicines are not right for every animal. Always confirm suitability and dose with your own veterinarian before giving any medication, especially the first time.