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Clemastine (Antihist-I® , Dayhist®, Tavist® )

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.
  • Clemastine 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. Clemastine has little to no effect on heart rate or stomach acid secretions.
  • Clemastine is available over the counter but should not be administered except under the supervision and 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: Antihist-I® , Dayhist® (Major), Tavist® (Novartis) and various generic preparations
  • Veterinary formulations: None

    Uses of Clemastine

  • Clemastine is used primarily to treat allergic symptoms, itching, and dermatitis. It is commonly used in the treatment of atopy.

    Precautions and Side Effects

  • While generally safe and effective when prescribed by a veterinarian, clemastine can cause side effects in some animals.
  • Clemastine 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 enlargement should not take clemastine.
  • Clemastine may interact with other medications. Consult with your veterinarian to determine if other drugs your pet is receiving could interact with clemastine. Such drugs include barbiturates and tranquilizers.
  • The most common adverse effects of clemastine are sedation, lethargy, vomiting, diarrhea and lack of appetite.

    How Clemastine Is Supplied

  • Clemastine is available in 1.34 mg and 2.68 mg tablets.
  • Clemastine 0.672 mg/5 ml oral syrup is also available.

    Dosing Information

  • Medication should never be administered without first consulting your veterinarian.
  • Clemastine is dosed in dogs at 0.025 – 0.05 mg per pound (0.5 – 0.2 mg/kg) PO two times daily. Some recent studies has suggested that clemastine is not as effective in dogs as previously thought. Further studies are needed.
  • In cats, clemastine is dosed at 0.67mg per cat every 12 hours for allergies. For atopic dermatitis, clemastine is dosed at 0.07 mg per pound (0.15 mg/kg) orally every 12 hours.
  • 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 or prevent the development of resistance.

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