Note: Even though chlorpheniramine is available over-the-counter, it could be unsafe for dogs with certain medical conditions or who are taking certain medication. Always consult with a vet before use. The typical dosage of chlorpheniramine for treating dogs is 4 – 8 mg every 8 to 12 hours, with a maximum dose of 1.1 mg/lb. Some vets recommend dosing by the dog’s weight at 0.9 – 1.7 mg/lb every 8 to 12 hours, but many agree on the upper limit of 1.1 mg/lb. Slightly lower amounts of around 0.5 mg/lb (up to 7 mg every 8 hours) could be used for sedation. Always follow the dosage recommended by your vet. 
Chlorpheniramine is considered quite safe for healthy dogs, unless an overdose is given. Be careful when treating a dog with medical conditions, because it might not be suitable for dogs with:
The drug is metabolized by the liver and excreted by the kidneys, so it’s wise to remain cautious when treating dogs with impaired liver or kidney function. The drug’s half-life may also be longer in these cases. Overdoses can cause severe and dangerous side effects, seek immediate veterinary help if an overdose has been given. If overdose-induced seizures occur do not administer diazepam (Valium) or barbiturates. Pregnancy/Nursing: Avoid use in pregnant or nursing animals.
The drug could intefere with the effects of anticoagulants, and the duration of antihistamine effects can be prolonged when given with MAOIs.
It is not a treatment for the actual mast cell tumors themselves, but is sometimes used for adjunctive therapy to control the increased amounts of histamine in the body. It is also occasionally used as a sedative.
Overdose: An overdose could cause seizures (do not treat these with barbiturates or Valium), coma, difficulty breathing, and even death in the most severe cases. If you suspect an overdose, immediately call your vet or the ASPCA poison control hotline on (888) 426-4435. If the medicine was given by mouth, you will usually be asked to make your dog vomit if he is alert and stable, and following this, activated charcoal is sometimes given. If seizures occur, Phenytoin is usually given intravenously. DO NOT act before you have spoken to a veterinary professional or ASPCA poison control specialist. Sources Dr. R. Rosychuk Dr. M. Papich Plumb’s Veterinary Drug Handbook (sixth edition)
When should I give my dog Piriton?
You should not give your dog Piriton unless you have consulted your vet and they have given specific advice surrounding your dogs needs. You should consult your vet if your dog is suffering from an allergic reaction and displaying any of the symptoms listed above.
A vet will know the correct cause of action for a dog with allergies.
Are there any side effects of Piriton for dogs?
While there are few side effects with the drug, its always important to know what to look out for.
1. Drowsiness: Piriton is an antihistamine, so it can make your dogs drowsy in the same way it does humans. Before giving it to your pup, make sure the dosage is correct.
2. Diarrhoea: Dogs react differently to different medications, so you may find your pup has an upset stomach. If this continues, its advised you book an appointment with your vet.
3. Dry mouth: Another side effect you may spot is your dog having a dry mouth. If this is the case, ensure your dog has plenty of fresh water to drink throughout the day.
4. Nausea/vomiting: Often, dogs will vomit as an initial reaction to something. This shouldnt last long, but if it does persist, stop giving the tablets to your dog and seek advice from a vet.
5. Changes in behaviour: You may notice a change in your dogs behaviour. Again, this should also pass but if you are worried its advised you seek professional advice from your vet.
Can I Give My Dog Piriton?
If your dog is suffering from hay fever, insect bites or a different allergic reaction then ask a vet first to make sure Piriton is safe and in what dosage you can give your dog Piriton.
Piriton is generally safe but seek veterinary advice for the correct dosage – usually three times daily – as well as any Piriton side effects to look out for.