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
Patient and units
The dose ladder differs by species, and the body weight can be entered in kilograms or pounds.
Weight, antihistamine and the two safety gates
This calculator is live: the dose is recalculated as soon as anything in this section changes.
Two safety gates before you see the dose
Tick both once you have genuinely checked. Not a formality.
Antihistamine dose estimate
Per-dose amount with the tablet and liquid equivalents, the frequency and the low-end 24-hour ceiling.
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.
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
Are antihistamine doses for dogs based on weight?
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.
Which antihistamines make pets drowsy?
Diphenhydramine, chlorphenamine and clemastine are sedating. Cetirizine is minimally sedating, while loratadine and fexofenadine are considered non-drowsy.
Can I give my dog Zyrtec-D or Claritin-D?
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 antihistamine can I give my 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.
What are the signs of an antihistamine overdose?
- 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.
How much Benadryl can I give my dog?
The chart on this page lists diphenhydramine (Benadryl) at 2 to 4 mg/kg for dogs and cats, every 8 to 12 hours, and it causes drowsiness. Enter your pet’s weight above to see the per-dose amount and the tablet or liquid equivalent. Use a plain, single-ingredient product, check that liquids and chewables are xylitol free, and confirm the dose with your vet first.
How much Zyrtec or Claritin can I give my dog?
Cetirizine (Zyrtec) and loratadine (Claritin) use flat dose ranges rather than a dose per kilogram. The chart on this page lists 5 to 10 mg once daily for a typical dog for both. For cats it lists cetirizine at 5 mg once daily, and loratadine only after you ask your vet. Avoid any “-D” version, which may contain pseudoephedrine.
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