Histamine is a mediator of inflammation released by inflammatory cells when they come into contact with certain triggers (allergens). Histamine is a normal part of the inflammatory cascade, a vital part of the body’s defense. But histamine and other inflammatory mediators cause the inflammation, swelling, reddening and itching seen in allergic reactions. These signs are part of the body’s attempt to cope with exposure to an allergen.

Antihistamines are medications used to counteract histamines and moderate this response. They work by competing with histamine for uptake at the histamine receptors on sensitive cells in the respiratory tract, intestines, blood vessels, and skin, and so dampen the response. Your vet may use an antihistamine as a treatment for hypersensitivity and allergic disorders. The particular antihistamine selected by your vet depends on their preference, the specifics of the condition being treated, and the horse.

Antihistamines are given orally and by injection. Oral pyrilamine maleate and tripelennamine are found in some over the counter commercial antihistamines. These are usually granular or powdered, intended to be given mixed with feed. These products have some use in preventative maintenance for horses with low grade allergies or allergic components of disease. They are generally considered safe, but are of questionable value.

Extra-label use of human antihistamines is also common in equine practice. Examples of these medications include hydroxyzine, diphenhydramine (Benadryl) and cetirizine, among many others. Cetirizine was shown NOT to be effective for treatment of Sweet Itch in horses. My own experience with this medication has been somewhat disappointing.

On occasion my clients use 5-8# of the 25 mg diphenhydramine (Benadryl) tabs in an emergency and report that hives decreased. Diphenhydramine is not FDA approved for use in horses, and I do not recommend that you proceed with this treatment without first talking to your vet.

Generally, oral antihistamines take longer to act than injectable preparations and are mostly used to prevent recurrence of minor itching, hives (urticaria) and swelling due to allergic skin problems, as well as low grade respiratory and ophthalmic allergy. There are also several injectable commercial antihistamines that are labeled for use in horses. These contain pyrilamine maleate and/or tripelennamine and act more rapidly given by injection. These are commonly used for more serious allergic reactions like hives, or facial swelling due to allergic response from an insect sting.

Generally, antihistamines are used less frequently in horses than corticosteroids. In some cases, they are not as effective. However, one advantage is that antihistamines have fewer side effects, the most important of which is laminitis. So antihistamines are valuable in lessening the frequency and/or dosage of corticosteroids when used concurrently.