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Chlorpheniramine Maleate (Chlor-Trimeton)
Revised: January 01, 2023
Published: January 01, 2001

(For veterinary information only)

The size of the tablet/medication is NOT an indication of a proper dose. Never administer any drug without your veterinarian's input. Serious side effects or death can occur if you use drugs on your pet without your veterinarian's advice. 

It is our policy not to give dosing information over the internet.

Brand Name: Chlor-Trimeton

Available in 4 mg tablets, 12 mg extended-release tablets, and oral syrup


Histamine is an inflammatory biochemical that causes skin redness, swelling, pain, increased heart rate, and blood pressure drop when it binds to one of many H1 receptors throughout the body. Histamine is an important mediator of allergy in humans, hence a spectacular array of different antihistamines has proliferated. Histamine, perhaps unfortunately, is not as important a mediator of inflammation in pets, which means the results of antihistamines are not as reliable in pets. Still, there are situations where antihistamines can be useful in dogs and cats.

How this Medication is Used

Chlorpheniramine maleate has several important effects and thus several uses. Most obviously, this medication is an antihistamine and it is used for acute inflammatory and allergic conditions such as:

Chlorpheniramine maleate is frequently included in antihistamine trials for allergic skin diseases. It is not one of the more effective antihistamines in dogs but is one of the most reliably effective antihistamines in cats (in one study, 73% of itchy cats responded). Its availability and inexpensiveness make it worth trying in many cases. Antihistamines are best used in a long-term setting as a preventive rather than in an acute setting where inflammation is already active.

Mast cell tumors are tumors involving cells that contain granules of histamine. Patients with mast cell tumors experience chronic inflammatory symptoms due to circulating histamine. Antihistamines such as chlorpheniramine maleate may be helpful in controlling inflammation when given long-term.

Chlorpheniramine maleate causes drowsiness in animals just as it does in people and can be used as a mild tranquilizer. Some argue that it is the drowsiness side effect that makes this medication appear to be helpful in itch management (i.e. patients scratch less because they are sleeping more.)

Convenient dosing, tablet size, and tablet strength make it a common choice in cats; however, its bitter taste may make its use problematic for cats.

Chlorpheniramine maleate is typically given 2-3 times daily. It can be given with or without food. If a dose is accidentally skipped, do not double up on the next dose. Simply give the dose when it is remembered and time the next dose accordingly.

Side Effects 

With so many possible uses of this medication, it is difficult to separate out a side effect from a primary effect. Drowsiness is generally regarded as an undesirable side effect but that depends on the circumstances. Mild sedation could be desired.

At doses higher than the recommended dose, human patients complain of dry mouth and experience difficulty with urination. Animal patients experiencing dry mouth may drink more water.

Chlorpheniramine maleate is famous for its bitter taste. Often the pet (especially cats) will tolerate the medication for a period of time but ultimately refuse to take it or even show salivation in response to administration. In such cases, it may be best to try a different medication.

Interactions with other Medications 

In the treatment of allergic skin disease, antihistamines are felt to synergize with omega-3 fatty acid supplements and, as a general rule for this condition, it is best to use these medications together.

Chlorpheniramine maleate should not be used with additional tranquilizing medications.

These combination products should not be used in animals. This antihistamine is used in an assortment of human products where it is combined with pain relievers and decongestants. These “combination” products should not be used in animals.

Concurrent use of chlorpheniramine with monoamine oxidase inhibitors (such as Amitraz, a tick-killing product, and selegiline, usually used to combat cognitive dysfunction) can increase the likelihood of dry mouth, constipation, and difficulty with urination.

Concerns and Cautions 

When using an antihistamine to prevent an allergic reaction (such as a vaccine reaction) the antihistamine works best when given prior to the allergen.

This medication will interfere with allergic skin testing. Check with your veterinary dermatologist regarding how far in advance this medication should be withheld.

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