What is an antihistamine?

An antihistamine is a medication that is used to treat allergies or allergic reactions. As its name suggests, an antihistamine works by opposing (anti) the effects of histamine. Histamine is a chemical released by certain immune system cells following exposure to the substance that causes an allergic reaction (an allergen).

How antihistamines work
When histamine is released, it binds to cells at sites called receptors. The cells that line the nose and throat then swell and leak fluid—a defensive measure aimed at getting rid of the allergen. The allergy sufferer experiences this process as nasal congestion, runny nose, sneezing, and itchy eyes and throat.

Antihistamines block the effects of histamine by attaching to the receptors before histamine can bind to them, thereby preventing allergy symptoms. It’s important to take an antihistamine in advance—preferably 2 to 5 hours before exposure to a known allergen. The allergen might be pollen, animal dander, or something else. If exposure is on-going, it’s advisable to take antihistamines on a regular basis.

Types of antihistamines
Antihistamines are available as tablets, capsules, nasal sprays, liquids, and by injection. They come in both over-the-counter (OTC) and prescription formulations. Most OTC preparations affect histamine receptors everywhere in the body, including the brain. That’s why they cause drowsiness. The newer “second generation” antihistamines, however, are non-sedating. With either category of antihistamine, be sure to let your doctor know what other medications you’re taking.

While both OTC and prescription antihistamines ease most allergy symptoms, they generally cannot relieve nasal congestion. For this reason, they may be combined with a decongestant.