As the name indicates, antihistamines are medications (available in tablet, capsule, liquid, and nasal spray forms or by injection) that counter the effects of histamine – a chemical substance released by the body as the result of injury or in response to an allergen.

Antihistamines block the effects of histamine and are most effective in controlling or alleviating symptoms of sneezing, runny nose, and itchy nose, eyes, and throat. However, these medications don’t usually reduce nasal congestion. As a result, they are frequently combined with a decongestant to relieve symptoms of congestion. In addition, antihistamines produce various side effects, depending on the type of product (over-the-counter or prescription), dosage level, and course of medication.

Studies show that the improvement of nasal symptoms produced by antihistamines has a beneficial effect on the functioning of lower airways associated with asthma.

If you wait to take antihistamines only after your symptoms begin, the medicine can’t reverse your reaction. Use antihistamines preventively instead. Take the medication two to five hours before you’re exposed to the allergen. If you maintain chronic contact with allergens, take antihistamines on a regular basis.

Nonprescription Antihistamines
The least costly and most common variety of antihistamine medications are first-generation nonprescription products that are available in over-the-counter (OTC) form. Hundreds of nonprescription antihistamine products line drugstore and supermarket shelves. Most of these products, however, are just different brand names for a few of the same active ingredients.

Although OTC antihistamines can relieve allergic rhinitis symptoms, they also produce side effects that can significantly interfere with your daily life. OTC antihistamines can cross from the bloodstream into your brain, where they affect histamine receptors in the central nervous system, resulting in drowsiness – the most serious and potentially dangerous side effect. Other side effects may include nasal stuffiness, dryness of the mouth and sinus passages, dizziness, gastrointestinal irritation, and urine retention.

Newer Prescription Antihistamines
Many people assume that OTC medications are somehow safer than prescription products. In the case of antihistamines, however, the reverse may be true. Due to significant advances in research since the development of first-generation antihistamines over 50 years ago, several of the newer, second-generation prescription antihistamines have fewer side effects. These newer products do not cross the blood-brain barrier, so they are nonsedating or only cause mild sedative effects.

Although second-generation antihistamines sometimes cost more than OTC products, they work longer and require only one or two doses per day to prevent or relieve allergic rhinitis symptoms. Overall, patients who use second-generation antihistamines usually experience much less disruption or impairment in their daily lives.

Reprinted from Allergies and Asthma for Dummies by William Berger, M.D. Available wherever you buy books.