Antihistamine Dose Calculator for Dogs & Cats
Safe starting doses for the antihistamines owners actually have in the cupboard: Benadryl, Zyrtec, Claritin and Piriton. Pick the drug, check the active ingredient, read the two gates. The gates are the part that keeps your pet alive.
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 four 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) | 4 to 8 mg | 1 to 2 mg | Dog q8 to 12 h, cat q12 h | Yes |
The detail
Why the doses do not all scale the same way
Diphenhydramine is dosed straight off body weight (2 to 4 mg/kg). Cetirizine and loratadine plateau, so they are given as a flat dose by size rather than a strict per-kg figure. A giant breed does not get a giant dose. Chlorphenamine scales with weight but caps out. This tool uses the right method for each drug rather than one formula for all.
Drowsy or not
Diphenhydramine and chlorphenamine are sedating, which is sometimes the point and sometimes a nuisance. Cetirizine is minimally sedating and loratadine is non-drowsy, so they are the better daytime choices when you do not want a flat, sleepy pet.
The combination-product trap
The single most dangerous mistake is grabbing a combination product. Anything ending in "-D" (Zyrtec-D, Claritin-D) contains pseudoephedrine. "PM", "Sinus", "Plus" and "Cold & Flu" packs often contain paracetamol or acetaminophen, ibuprofen, or phenylephrine. Paracetamol can kill a cat in a single small dose. If the box lists anything other than the one antihistamine, do not use it.
Which one for a cat
Cetirizine and chlorphenamine are the more sensible choices for cats. Diphenhydramine works but cats are more sensitive and can react the opposite way, either flat or agitated. Loratadine is not routinely recommended in cats. Whatever you use, the product must be single ingredient, and confirm the first dose with your vet.
Signs you gave too much
- Heavy sedation, or the opposite, hyperactive and agitated
- Fast heart rate, dilated pupils, dry mouth, wobbliness
- In a real overdose, tremors, seizures, trouble breathing
If you see these, or you realise you gave a combination product by mistake, ring a vet or an animal poison line straight away.
Prescription options exist too
Hydroxyzine and other antihistamines are stronger options your vet can prescribe and dose for your pet. They are not in this calculator on purpose, because they need a vet's direction. If the cupboard options are not cutting it, that is the conversation to have.
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.