Clemastine Fumarate

(for veterinary information only)


1.34 mg & 2.68 mg


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 a very 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 results of antihistamine therapy are not as reliable in pets.


Antihistamines have several important effects and thus several uses. They are most commonly used for acute inflammatory and allergic conditions such as:

Clemastine fumarate is frequently included in antihistamine trials for allergic skin disease. In these trials, several antihistamines are used sequentially in an effort to find one that is effective. As mentioned, a given antihistamine is not very likely to be effective but trying several in sequence increases the likelihood of finding one that works. In older studies, efficacy of clemastine was found to be helpful in at least 30% of itchy dogs and 50% of itchy cats. Other studies have found higher percentages. Popularity of this medication in veterinary use rose. More recent studies suggest that clemastine is poorly absorbed into the body at least in dogs leaving previously optimistic findings unexplained and use has become more controversial. Still, clemastine may still be worth a try.

Clemastine fumarate is used on a twice daily basis for most animal patients, whereas many other antihistamines must be given 2 or 3 times daily. This adds a substantial convenience factor to the use of this medication.

Mast cell tumors are especially inflammatory tumors which contain fragile granules of histamine. Patients with mast cell tumors experience chronic inflammatory symptoms due to circulating histamine. Antihistamines such as clemastine fumarate may be helpful when given long term though less expensive medications such as diphenhydramine (Benadryl) tend to be more commonly used.

Clemastine fumarate (like diphenhydramine) is of the ethanolamine group of antihistamines. If a trial of antihistamines is being conducted and one antihistamine is found ineffective, in general, it is a good idea to try one of a different class to avoid a repeat failure.


Clemastine fumarate causes drowsiness in animals just as it does in people.

At doses higher than the recommended dose, human patients complain of dry mouth and experience difficulty with urination. While animal patients are unable to verbally complain about such side effects, animals experiencing dry mouth may be seen drinking more water than usual. If this occurs, a different antihistamine from a different antihistamine class may not have these side effects in the same patient.

Diarrhea is the most common side effect in cats.


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.
If clemastine fumarate is used with additional tranquilizing medications, the tranquilizing effect could be greatly enhanced.


Clemastine fumarate is available in a common over-the-counter product called Tavist-D.
The “D” stands for decongestant and refers to the addition of the medication
phenylpropanolamine to the medication. It is best not to use this product for pet use
as the dose of phenylpropanolamine may not be appropriate.

Clemastine fumarate appears to be safe for use in pregnancy but it does cross into mother’s milk in lactating animals. This means that nursing young will be medicated inadvertantly.

This medication will interfere with allergic skin testing. If you are considering this useful diagnostic test, it is important to check with your veterinary dermatologist regarding how far in advance this medication should be withheld before the test.

Page last updated: 7/19/2011