Some of the medications used to treat respiratory allergy symptoms include antihistamines, decongestants, nasal sprays and eyedrops. This article focuses on antihistamines, which may also be used to treat the symptoms of drug and insect allergies.

Antihistamines - the most common treatment for hay fever - work by blocking the action of histamine, an inflammatory substance released when your immune system encounters an allergen, such as pollen or mold.

These drugs - generally taken as a pill, liquid, injection or nasal spray - are available over-the-counter (OTC) and by prescription to help relieve or prevent the sneezing, itchy eyes and throat, and postnasal drip that histamine may cause. Typically, they’re not quite as effective for a stuffy nose.

Antihistamine pills are commonly used for their convenience, although they may be more likely to produce drowsiness and dry mouth than are antihistamine eyedrops or nasal sprays. A prescription antihistamine nasal spray, such as azelastine (Astelin), has some effect on reducing nasal congestion. Prescription antihistamine eyedrops can help ease eye itching and redness due to allergies. OTC eyedrops also may help relieve minor itching and redness.

Oral antihistamines are available OTC and by prescription. A wide variety of brands of OTC products such as diphenhydramine (AllerMax, Benadryl, others), chlorpheniramine (Aller-Chlor, Chlor-Trimeton, others) and clemastine (Antihist 1, Tavist) are available and are usually relatively inexpensive. However, these products can cause side effects such as drowsiness and dry mouth. Loratadine (formerly Claritin by prescription) is a newer OTC antihistamine that causes fewer problems with drowsiness and dry mouth. Avoid antihistamines if you have certain types of glaucoma or an enlarged prostate, as they can aggravate these conditions. Also tell your doctor if you’re pregnant, breast-feeding or on a restricted diet because some antihistamines may not be safe for you.

Prescription oral antihistamines that cause fewer problems with drowsiness and dry mouth include fexofenadine (Allegra) and desloratadine (Clarinex). Cetirizine (Zyrtec) is another example. However, cetirizine does have a mild sedating effect in some people. Older prescription antihistamines, although effective, generally have more side effects such as drowsiness and dry mouth. These products, however, are less expensive and may be covered by more insurance companies than are the newer products.

Talk to your doctor or pharmacist before taking any medications.