Chlorpheniramine is a type of antihistamine which is sometimes used to treat the symptoms of allergies and itching in dogs. It is a first generation H1 antagonist which acts similar to diphenhydramine, and like diphenhydramine it does cause drowsiness, but to a slightly lesser degree. This makes it inferior for sedation, but useful when you want to relieve the symptoms of allergies without causing sleepiness, or when lighter sedation is required.

It has a good safety profile when used in healthy dogs.

Recommended Dosage

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.

Chlor-Trimeton allergy reliefBranded Forms:

Branded formulations include Chlor-Trimeton and Piriton. If you’re using a branded medicine check the packaging to make sure it contains no active ingredients other than chlorpheniramine.

Sustained-Release Products:

If you have sustained-release chlorpheniramine capsules you can pop them open and pour the contents onto your dog’s food, but make sure he eats it before the medicine dissolves. Avoid letting him chew on them and avoid crushing the capsules.

Example: A 50 lb dog with symptoms of itching could be given 8 mg every 8 to 12 hours, depending on the dosage protocol being used, and should not be given more than 55 mg.


Is It Safe?

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:

  • Angle-closure glaucoma
  • Bladder neck obstruction
  • Chronic obstructive pulmonary disease (COPD)
  • Enlarged prostate
  • Gastric outflow obstruction
  • Heart disease
  • Hypertension
  • Hyperthyroidism
  • Kidney or liver impairment

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.

Safety Guidelines

Following these guidelines will help you to use this medicine in the safest way possible:

  • Only use under the guidance of a vet
  • Tell the vet about any medical conditions your dog has and any medicines/supplements you are giving him
  • Practice caution when using with anticoagulant drugs (e.g. warfarin) or MAOI drugs (e.g. amitraz)
  • Practice caution when treating dogs with liver disease, as the drug is metabolized by the liver

The drug could intefere with the effects of anticoagulants, and the duration of antihistamine effects can be prolonged when given with MAOIs.

What Is It Used For?

Chlorpheniramine is used to treat the following conditions:

  • Allergies
  • Itching
  • Mast cell tumors

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.

Side Effects

Chlorpheniramine can cause the following side effects:
Most Common

  • Sedation

Less Common

  • Vomiting
  • Diarrhea
  • Changes in appetite
  • Urine retention
  • Dry mouth


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.


Dr. R. Rosychuk
Dr. M. Papich
Plumb’s Veterinary Drug Handbook (sixth edition)