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Chlorpheniramine Maleate (Chlor-Trimeton®)

By: Dr. Dawn Ruben

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  • Histamine is a chemical that is released in the body in response to inflammation or allergy. This chemical travels throughout the body searching for specific histamine receptors (targets on cells). Once attached to the receptors, histamine will cause swelling, itchiness and other symptoms associated with an allergic response.
  • There are two types of histamine receptors: H1 and H2. H1 receptors affect small blood vessels and smooth muscles. When histamine attaches to the H1 receptors, the small blood vessels dilate and fluid begins to leak out. This results in tissue swelling and itchiness. In addition, the smooth muscles lining the small airways constrict, causing tightness and some breathing difficulty. H2 receptors affect heart rate and stomach acid secretions. When histamine attaches to H2 receptors, the heart rate increases and stomach acid secretions are increased, potentially raising the risk of developing ulcers.
  • Drugs that block the effects of histamine are called antihistamines. There are a number of drugs demonstrating antihistamine effects; some are useful in allergies, others for preventing excessive stomach acid. The effects of the antihistamine depend on whether it binds with the H1 receptors or H2 receptors. There are few drugs that affect both types of receptors.
  • Chlorpheniramine maleate is one type of antihistamine that inhibits the action of histamine, particularly its effect on H1 receptors. This results in a reduction or prevention of swelling and itchiness. Chlorpheniramine has little to no effect on heart rate or stomach acid secretions.
  • Chlorpheniramine is available over the counter but should only be given under the guidance of a veterinarian.
  • This drug is not approved for use in animals by the Food and Drug Administration but it is prescribed legally by veterinarians as an extra-label drug.

    Brand Names and Other Names

  • This drug is registered for use in humans only.
  • Human formulations: Chlor-Trimeton® (Schering) and various generic preparations
  • Veterinary formulations: None

    Uses of Chlorpheniramine

  • Chlorpheniramine is used primarily to treat allergic symptoms and allergic reactions.
  • Sedation of a mild nature is another potential consequence of treatment.

    Precautions and Side Effects

  • While generally safe and effective when prescribed by a veterinarian, chlorpheniramine can cause side effects in some animals.
  • Chlorpheniramine should not be used in animals with known hypersensitivity or allergy to the drug.
  • Animals with glaucoma, lung disease, heart disease, high blood pressure and prostate gland enlargement should not use chlorpheniramine.
  • Chlorpheniramine may interact with other medications. Consult with your veterinarian to determine if other drugs your pet is receiving could interact with chlorpheniramine. Such drugs include barbiturates, tranquilizers and heparin.
  • The most common adverse effects of chlorpheniramine are sedation, lethargy, vomiting, diarrhea and lack of appetite.

    How Chlorpheniramine Is Supplied

  • Chlorpheniramine is available in 2 mg, 4 mg, 8 mg and 12 mg tablets.
  • Chlorpheniramine 2 mg/5 ml oral syrup is also available.
  • Chlorpheniramine 10 mg/ml and 100 mg/ml injectable form is also available.

    Dosing Information

  • Medication should never be administered without first consulting your veterinarian.
  • Chlorpheniramine is dosed for dogs at 4 to 8 mg (max 0.5 mg/kg) two times per day. For cats at 2 to 4 mg per cat once a day.
  • The duration of administration depends on the condition being treated, response to the medication and the development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your pet feels better, the entire treatment plan should be completed to prevent relapse.

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