Piriton for Dogs: Side Effects To Be Aware Of

dog piriton side effects

Vets and dog owners have been giving dogs Piriton for over 50 years. It’s been used successfully to treat a range of conditions including allergies, hay fever, insect bites, and bee stings. But, it’s still a medicine that designed for humans, and as with anything of this nature, some people, and dogs, can suffer adverse reactions.

The fact is, some dogs might suffer side effects from Piriton. This can happen even with dogs who have never shown any ill effects of the medicine. Factors that can lead to a bad side effect include the dosage amount being wrong, the age and health condition of the dog, or whether the dog is already on any other medication.

This is why you should always consult with your vet before giving a dog any medicine that has not been prescribed for them.

The reason I’ve put this guide together is because a good friend of mine gave his dog Piriton and it made his dog very ill. After seeing this, I thought it prudent to publish a list of what the possible side effects of giving Piriton to dogs are. They are rare, but you need to be aware of them.

Side effects of Piriton in dogs?

Before we go on, it’s important to understand that dogs can overdose on Piriton. It’s what’s known as an “older” or “first generation” antihistamine and can cause drowsiness and work as a sedative if you get the dosage wrong.

Symptoms of a chlorphenamine overdose

The main element of Piriton is chlorphenamine. According the UK’s NHS website, humans can suffer the following symptoms when they have taken too much chlorphenamine contained in Piriton.

  • Feel very sleepy.
  • Feel nervous or restless.
  • Feel or be sick (nausea or vomiting).
  • Get blurred vision.
  • Get a very fast, uneven or pounding heartbeat (palpitations).

The NHS go on to say that in serious cases, humans can have fits or become unconscious with the possibility of emergency treatment in hospital.

Given that we now know that, imagine what an overdose of Piriton containing chlorphenamine could do to a dog, who is much smaller than an adult human?

The possible side effects of giving Piriton to dogs

Now we’ve got the scariest stuff out the way, we can now look at the side effects, which are still scary of course! To find out, I read multiple veterinary blogs to find these Piriton side effects in dogs. It’s worth mentioning that most website extol the virtues of Piriton for dogs, providing the dosage is correct.

But you still should be extremely cautious and only give your dog Piriton once you’ve had the go-ahead from your vet.

Here are some possible side effects that have been reported online, although not every dog will get them.

1. Vomiting and nausea

Some dogs will become nauseous and vomit on Piriton. The theory is that antihistamines block certain receptors in the part of the brain that creates nausea in response to chemicals getting into the body.

2. Constipation or diarrhoea

All medicines can play havoc with a dog’s digestive system, and some dogs will have the rather unfortunate Piriton side effect of becoming constipated or the opposite reaction of diarrhoea.

3. Loss of appetite

A secondary side effect of the digestive upset will be your dog’s not wanting to eat.

4. Mild straining to urinate

Antihistamines like Piriton can tighten the muscles around your dog’s urethra, making him possibly need to strain in order to pee.

5. Excessive thirst and dry mouth

There is also some evidence that antihistamines can make humans feel dried out, making them thirsty… and we have to assume that dogs might experience a similar side effect to the drug.

6. Sleepiness

One of the more common side effects of giving your dog Piriton will be drowsiness. Piriton tablets contain antihistamine which has a sedating affect. Because of this, you need to be very wary about how much you give to small dogs.

More serious side effects of Piriton in dogs

Those are the milder side effects of giving Piriton to dogs, and here are some very serious, albeit rare possibilities.

7. Seizures and muscle tremors

The chlorphenamine in Piriton isn’t suitable for dogs that suffer with epilepsy or any other health problems that can result in fits.

8. Abnormal breathing and incoordination

Piriton could also cause an allergic reaction in a dog, leading to swelling of the throat or nasal passage, resulting in breathing difficulties and incoordination.

And lastly…

As discussed, the main component of Piriton is a drug called Chlorpheniramine maleate. The VCA Hospitals website give this warning on it when used on dogs:

“Chlorpheniramine maleate should not be used in pets that are allergic to it or other similar antihistamines. Use cautiously in pets with closed-angle glaucoma, enlarged prostate, hyperthyroidism, heart disease, high blood pressure, gastrointestinal or bladder obstruction, or obstructive lung disease (COPD). Use cautiously in geriatric, pregnant or lactating pets, or dogs that are working dogs.” (view source) 

VCA also go on to explain that there are certain medications your dog should not take Piriton with: 

“Anticoagulants, MAOIs, phenytoin, or central nervous system depressants. Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking. This medication can also interact with skin allergy testing; discontinue this medication at least 2 weeks before allergy testing.”

What does Piriton help with

Vets have said that the following conditions in dogs can be helped with this antihistamine tablet.

  • Allergies
  • Anxiety
  • Bee stings
  • Dust mite hypersensitivity
  • Eye irritation
  • Fleas
  • Hay fever
  • Insect Bites
  • Itchiness
  • Runny noses
  • Scratching

Some people and vets even use Piriton to help with adder bites. You can read more about that in my guide to Piriton and adder bites in dogs.

Is Piriton dangerous for dogs then?

In most cases, no, Piriton is not dangerous in dogs, with thousands of dogs around the world having it every week with no bad side effects.

However, a small percentage of dogs will suffer a bad reaction. So, Piriton can be dangerous to dogs, particularly when dosages are given incorrectly leading to an overdose.

Chemists cannot sell you Piriton if it’s for your dog

Before you do rush out and buy Piriton tablets for your dog, please appreciate that chemists cannot legally sell a person Piriton if it is to be given to a dog. From a legal perspective, if you want to give your dog Piriton for an adder bite you would have to get it as a vet’s prescription. Over the counter Piriton is not licensed for animal use.

You should also never give your dog Piriton unless it’s under strict guidance of your vet. Call them before you give it to your dog and ask them how you should proceed.

Dosage suggestions 

The dosage amounts that various websites give on Piriton for dog bites all are reasonably consistent in their approach:

  • 2m to 4mg for a smaller dog bred (typically half to one Piriton tablet).
  • 4mg to 8mg for a larger dog breed (typically one to two Piriton tablets).

However, you still need to check with your vet first on what the correct dosage will be for your dog. Proper dosage levels will depend on the size, weight, and breed of your dog, plus their overall health and existing medications.

My assumption is that you need to be particularly careful on dosage with very small dogs, including toy breeds and puppies. 

Overdosing could result in a seizure, please talk with a vet first. 


Dog owners will often give their pets medicines that have not been designed for animal use. In fact, there was a survey in the Vet Times which found the following: 

“In a survey of 1,000 British cat and dog owners, almost 10% admitted giving human medicine to their pets. The products concerned ranged from antihistamine creams to a wide range of pain killers and antiseptic creams.” 

You need a subscription to read the full article which goes on to explain that nearly 4% of the British public give antihistamines to their dogs. 

Whilst Piriton is widely used, and even recommended by some vets, the administering of these tables needs to be done under a professional guidance.

The bottom line is, always consult with your vet before giving your dog Piriton to reduce the risk of any dangerous side effects.

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Marc Aaron

I write about the things we've learned about owning dogs, the adventures we have, and any advice and tips we've picked up along the way.

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