Antihistamines for Dogs: Piriton, Piriteze, Clarityn and Benadryl Compared
Which UK antihistamines dogs can have, the dose for each, and the hayfever products to keep away from them
Last updated: 29 October 2026
UK vets sometimes recommend three human antihistamines for dogs: chlorphenamine (Piriton), cetirizine (Piriteze) and loratadine (Clarityn). Each has a different dose, none is licensed for dogs, and products with a decongestant are dangerous.
UK Benadryl is not the same as the Benadryl in American advice: none of the UK range contains diphenhydramine. Check the active ingredient on the box, use the calculator for that drug, and ask your vet before giving any.
The Three Antihistamines Vets Use for Dogs
| Drug | UK brands | Dog dose | How often | Drowsiness |
|---|---|---|---|---|
| ChlorphenaminePiriton calculator | Piriton | 0.2–0.4 mg/kg, max 8 mg | Every 8–12 hours | Often |
| CetirizineCetirizine calculator | Piriteze, Zirtek, Benadryl One A Day | 0.5–1 mg/kg, max 20 mg | Once a day | Less likely |
| LoratadineLoratadine calculator | Clarityn, supermarket one-a-day | 0.25–0.5 mg/kg, max 20 mg | Once a day | Less likely |
Loratadine shows the lower half of the published range as a cautious guide. Your vet may advise a different dose.
Antihistamine Dose Chart for Dogs by Weight
Amount per dose in mg. Give only one antihistamine, at the frequency in the table above. Open each calculator for tablets and ml of syrup.
| Dog’s weight | Piriton (chlorphenamine) | Piriteze (cetirizine) | Clarityn (loratadine) |
|---|---|---|---|
| 5 kg (11 lb) | 1 – 2 mg | 2.5 – 5 mg | 1.3 – 2.5 mg |
| 10 kg (22 lb) | 2 – 4 mg | 5 – 10 mg | 2.5 – 5 mg |
| 15 kg (33 lb) | 3 – 6 mg | 7.5 – 15 mg | 3.8 – 7.5 mg |
| 20 kg (44 lb) | 4 – 8 mg | 10 – 20 mg | 5 – 10 mg |
| 30 kg (66 lb) | 6 – 8 mg | 15 – 20 mg | 7.5 – 15 mg |
| 40 kg (88 lb) | 8 mg | 20 mg | 10 – 20 mg |
Can I Give My Dog Benadryl? Why UK Benadryl Is Different
Much of the advice online is American, where Benadryl means diphenhydramine. No UK Benadryl product contains diphenhydramine, so American Benadryl doses do not apply to anything you can buy here as Benadryl.
| UK product | Contains | For dogs? |
|---|---|---|
| Benadryl Allergy Relief | Acrivastine 8 mg | No: there is no established dog dose |
| Benadryl Allergy Relief Plus Decongestant | Acrivastine and pseudoephedrine 60 mg | No: pseudoephedrine is dangerous |
| Benadryl Allergy One A Day | Cetirizine 10 mg | Only at the cetirizine dose, if your vet advises it |
| Benadryl Allergy Children’s solution | Cetirizine 1 mg/ml | Only at the cetirizine dose, if your vet advises it |
One Benadryl Allergy Relief Plus Decongestant capsule contains enough pseudoephedrine to cause signs such as agitation and a racing heart in dogs up to about 12 kg, and it can be fatal for dogs up to about 6 kg. In the UK, diphenhydramine is mainly sold as a sleep aid (such as Nytol) and in night-time cough and cold remedies, some of which also contain paracetamol or a decongestant. Do not give these to your dog unless your vet has prescribed them.
Can Dogs Have Hayfever Tablets?
Sometimes, but it depends entirely on what is in them. Supermarket and pharmacy hayfever tablets usually contain one of three drugs, and the name on the front does not always tell you which:
- Cetirizine, often sold as “hayfever and allergy relief”: see the cetirizine calculator
- Loratadine, often sold as “one a day”: see the loratadine calculator
- Acrivastine (Benadryl Allergy Relief): no established dog dose, so avoid it
Never give a hayfever product that also treats a blocked nose: those contain a decongestant such as pseudoephedrine, which is dangerous for dogs.
Never Give Your Dog
- • Any antihistamine combined with a decongestant (pseudoephedrine or phenylephrine)
- • Cold and flu or night-time remedies that mix an antihistamine with paracetamol, ibuprofen or other medicines
- • Liquids, melts or chewables containing xylitol (E967), which is poisonous to dogs
- • Two antihistamines at once, unless your vet has told you to
Do Antihistamines Work for Dogs?
They can help with mild allergic reactions such as hives or insect bites, but they are much less reliable for long-term itchy skin. The MSD Veterinary Manual says responses “vary considerably” and that more than one may need to be tried. If your dog keeps scratching, ask your vet about licensed veterinary allergy treatments, and see why is my dog so itchy?
Emergency signs: swelling of the face or throat, difficulty breathing, collapse or repeated vomiting after a sting or bite need a vet straight away. Do not wait to see if an antihistamine works.
Frequently Asked Questions
Which antihistamines can dogs have?
Vets in the UK sometimes recommend three human antihistamines for dogs: chlorphenamine (Piriton), cetirizine (Piriteze, Zirtek) and loratadine (Clarityn). None are licensed for dogs, so only give one if your vet has advised it, at the dose for your dog’s weight, and never a product that also contains a decongestant.
Can I give my dog Benadryl in the UK?
Not the way US advice suggests. US Benadryl is diphenhydramine, but no UK Benadryl product contains it. Benadryl Allergy Relief is acrivastine, which has no established dog dose, and Benadryl Allergy Relief Plus Decongestant also contains pseudoephedrine, which is dangerous for dogs. Benadryl One A Day is cetirizine, the same drug as Piriteze.
Can dogs have hayfever tablets?
Some UK hayfever tablets contain cetirizine or loratadine, which vets sometimes use for dogs at a dog-specific dose. Others contain acrivastine or a decongestant, which should not be given to dogs. Check the active ingredient on the box and ask your vet before giving any.
Which antihistamine is best for dogs?
There is no single best one. Dogs respond differently, and the MSD Veterinary Manual notes that several may need to be tried to find one that works. Antihistamines are also less effective for long-term itchy skin than licensed veterinary allergy treatments, so ask your vet if the itching keeps coming back.
Can I give my dog Piriton and Piriteze together?
Not unless your vet has told you to. They are different antihistamines, and giving two together adds to the side effects such as drowsiness. Use one, at the dose for that drug.
Which antihistamine products are dangerous for dogs?
Any product that also contains a decongestant such as pseudoephedrine or phenylephrine, including Benadryl Allergy Relief Plus Decongestant and many cold and flu remedies. Also avoid liquids or melts containing xylitol, and night-time remedies that combine an antihistamine with paracetamol or other medicines.