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• Chlorpheniramine maleate, commonly known as Chlor-Trimetron® or Chlor-tabs®, is a type of antihistamine drug most commonly used to control itching or as a mild sedative in dogs and cats. It is categorized as a first-generation antihistamine.
• 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 a 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 approved for use in some countries, such as Australia, UK, and Canada, but in the United States, chlorpheniramine is not approved for the use in animals by the Food and Drug Administration (FDA). It can only be legally prescribed by veterinarians as an extra-label drug.

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• This drug is registered for use in humans and animals.
• Human formulations: Aller-Chlor®, Chlo-Amine®, Chlor-Trimeton®, Chlor-tabs®, Pharbechlor® (Ulai), Piriton® (GlaxoSmithKline), and various generic preparations.
• Veterinary formulations: Histamil® (Troy), Histodine® (Le Vet Beheer), Iramine®, Mavlab, and Niramine® (Jurox).

Uses of Chlorpheniramine for Dogs and Cats

• Chlorpheniramine is used primarily to treat allergic symptoms and reactions to vaccines, blood transfusions, snake bites, bee and wasp stings, and insect bites. It can be used with other antihistamine drugs (such as hydroxyzine) or Omega-3 and 6 fatty acids to improve its effectiveness.
• The second most common use is as a mild sedative and to treat pets with motion sickness.
• To avoid a post-surgical histamine response, it can be used prior to surgery in pets with mast cell tumors.
• It is used in pets with arthritis or asthma, often in combination with prednisolone.
• Use as a cough suppressant in dogs, in combination with other drugs, such as dextromethorphan and ephedrine, is possible.

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Precautions and Side Effects

• While generally safe and effective when prescribed by a veterinarian, chlorpheniramine can lead to side effects in some animals. Chlorpheniramine should not be used in animals with known hypersensitivity or an allergy to the drug. Symptoms of hypersensitivity included hives and facial swelling.
• Animals with glaucoma, lung disease (such as asthma, heart disease, high blood pressure, epilepsy, and hyperthyroidism), and prostate gland enlargement should not use chlorpheniramine. It should be used with caution in pets with liver disease and urinary retention.
• Safety and risks associated with chlorpheniramine use during pregnancy and lactation have not been established.
• The most common adverse effects of chlorpheniramine are sedation, lethargy, vomiting, diarrhea, lack of appetite, itching (pruritus), agitation, and keratoconjunctivitis sicca. Dry mouth is a common side effect in humans and may occur in pets, but is difficult to document. Some cats may experience agitation or excitement during administration.
• Overdoses or toxicities are possible, especially when chlorpheniramine is formulated with other ingredients such as pseudoephedrine or acetaminophen.

Drug and Laboratory Interactions

• Chlorpheniramine maleate may interact with other medications. Consult with your veterinarian to determine if other drugs your pet is receiving could interact. Such drugs include amitraz, amitriptyline, amphetamine, barbiturates, epinephrine, fatty acid supplements, heparin, phenytoin, pseudoephedrine, selegiline, and tranquilizers.
• Use of chlorpheniramine can interfere with results of skin antigen testing.

How Chlorpheniramine Is Supplied and Stored

• Chlorpheniramine is available in 2 mg, 4 mg, 8 mg, 12 mg, and 16 mg tablets. An extended-release product is available in 12 mg size.
• Chlorpheniramine maleate is available in 2 mg chewable tablets.
• It is also available as a 2mg/5 ml & 4 mg/5 ml oral syrup and 2 mg/mL oral drops.
• Chlorpheniramine 10 mg/ml and 100 mg/ml injectable form is also available.
• It can also be compounded into a transdermal gel by some compounding pharmacies.
• Tablets and oral syrup should be stored between 50°F – 85°F. Avoid freezing and hot humid conditions.
• IMPORTANT NOTE: Chlorpheniramine maleate is often combined with other drugs. It is important to carefully look at the label to evaluate active ingredients.

Dosing Information of Chlorpheniramine for Dogs and Cats

• Chlorpheniramine is available over the counter but should never be administered without first consulting your veterinarian. The dose and frequency recommended will depend on the condition being treated, underlying health problems, and use of concurrent medications.
• Chlorpheniramine maleate is dosed in dogs at 0.1 mg per pound to 0.4 mg per pound (0.2 to 0.5 mg/kg) every 8 to 12 hours. Another way Chlorpheniramine maleate is dosed in dogs is by the total dose. For example, most dogs will receive 4 to 12 mg (total dose) orally two to three times daily (every 8 to 12 hours). Dose should not exceed 0.25 mg/per pound (or 0.5 mg/kg).
• When used as a mild sedative, the dose used in dogs is 0.11 mg/pound (or 0.22 mg/pound) every 8 hours.
• The most common dose used in cats is 2 mg total dose per cat every 8 to 12 hours. The medication taste is often unpleasant for cats. In these cases, an alternative form of the medication (such as transdermal) or a different medication may be preferred.
• The extended-release product should never be split or crushed.
• Medication should be given at the same time each day. If you miss a dose and remember it within 3 hours, give it and then give the next scheduled dose as per the original schedule. If it has been longer than 3 hours since you missed a dose, give the next dose as originally scheduled. Do not give a double dose unless otherwise instructed by your veterinarian.
• 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.