During allergic reactions—including both reactions to seasonal airborne allergens like pollen and reactions to food in sensitive individuals—immune cells secrete a chemical messenger called histamine. This chemical causes the classical symptoms of allergies, including runny or stuffy nose, itchy eyes, sneezing and congestion. To cause these reactions, histamine must attach to receptors on body cells; antihistamines are pharmaceuticals that prevent histamine from doing so, thus alleviating allergy symptoms. There are several common antihistamine chemicals.
Diphenhydramine
One of the most common and oldest of antihistamines is diphenhydramine, sold under the brand name Benadryl. According to PubMed Health, a publication of the U.S. National Library of Medicine, diphenhydramine causes a number of side effects in addition to its antihistime action. Many antihistamines cause drowsiness, including diphenhydramine, which does this so predictably that it can be used as a sleep aid in addition to an antihistamine. In addition to its use in treating seasonal allergy symptoms, diphenhydramine is also the drug of choice in early treatment of severe food allergy reactions—it can help lessen the severity of food reactions that might otherwise lead to throat swelling and difficulty breathing.
Loratadine
The development of the so-called “second generation” antihistamines represented a significant stride forward in allergy treatment. Unlike many first-generation drugs, second-generation pharmaceuticals show more specificity for peripheral histamine receptors as opposed to central receptors, reducing side effects like sleepiness. Loratadine, sold under the brand name Claritin, is one such second-generation antihistamine. Loratadine is a heavy, chlorine-containing molecule. Because of the chemical’s affinity for very specific cells—particularly those most likely to be involved in fluid-secretion during allergy attacks—it causes few side effects. The most common effects reported tend to include headache, insomnia and dry mouth, largely because the drug by definition dries up secretions, which causes dryness and can lead to headache.
Fexofenadine
Another second-generation antihistamine with few side effects is fexofenadine, sold as Allegra. Available by prescription only, fexofenadine is used more to prevent than to treat allergy symptoms. The drug is typically prescribed to routine allergy sufferers, who are advised to take it daily to prevent allergy attacks. Unlike faster-acting, over-the-counter antihistamine drugs, fexofenadine doesn’t tend to work right away and is often less effective at stalling an active allergy attack. Like other second-generation antihistamines, side effects are much more mild than those associated with first-generation drugs and include general dryness of mucus membranes and headache.
