Oral antihistamines have been in use since the 1950s and play a major role in treating allergic rhinitis. This class of drugs blocks the histamine H1 receptor. They are most effective in reducing symptoms of sneezing, nasal and ocular itching, and nasal discharge. However, they have very little effect upon nasal congestion. These medica-tions are to be distinguished from histamine H2 receptor blockers, such as ranitidine and cimetidine, which are largely used to treat gastroesophageal reflux disease and peptic ulcers. Because H1 antihistamines must bind to the histamine receptor before histamine does, these drugs are most effective when used before coming into contact with a known allergen.
Patients are often concerned that these medications will stop being effective after several weeks or months of use; however, long-term controlled studies have shown that this is not the case. The older antihistamines, including chlorpheniramine, diphenhydramine, and clemastine, are usually taken two to three times per day and are available either alone or in combination with an oral decongestant such as pseudoephedrine. Unfortu-nately, these older medications pass readily through a membrane around the brain called the “blood–brain barrier” and into the central nervous system, causing sedation in adults and occasionally paradoxic stimulation in chil-dren. In addition, these older drugs do not specifically block histamine receptors but also block a number of other chemical receptors, most notably the cholinergic recep-tors. Blockage of these cholinergic receptors through-out the body often leads to constipation and dryness of the mouth and eyes, and may provoke aggravations of uri-nary blockage and glaucoma in the elderly.
There is a growing group of newer oral antihistamines, including loratadine, fexofenadine, cetirizine, deslorata-dine, and levocetirizine . These drugs have onsets of action within 1 to 2 hours and are all taken once daily. Loratadine and cetirizine are now available over the counter in both generic and proprietary formulations These newer antihistamines do not pass as readily through the blood–brain barrier and do not bind to the cholinergic receptors; for these reasons, they cause minimal or no sedation, con-stipation, or other effects noted with the older antihis-tamines. Many controlled studies have demonstrated that these medications are equally effective as the older agents. Antihistamines are also available as topical nasal sprays. Both azelastine and olopatadine have been shown to be at least as effective as oral antihistamines and begin to reduce symptoms as quickly as 10 minutes after administrationl.
Antihistamine
An agent that binds to the histamine receptor and blocks the effects of hista-mine on the body.