An antihistamine is a medication that treats allergy symptoms by inhibiting the activity of histamines in the body. An antihistamine designed for oral administration comes in the form of pills, capsules, liquids and chewable tablets, notes MedlinePlus.
Exposure to allergy-causing agents, such as ragweed, pet dander, pollen and dust mites, can trigger the release of chemicals called histamines in the body, explains WebMD. These compounds bind to the cells, which then induces fluid discharge and an inflammatory response. This can further lead to itching, watery eyes, sneezing and runny nose. Taking an antihistamine product keeps histamines from attaching to the cells to control or even prevent the onset of allergy symptoms.
Antihistamines are considered to be safe for adults and the majority of these products are also deemed safe for children age 2 years and above. However, there are known side effects of antihistamines that include drowsiness, lightheadedness, dry mouth, queasiness and retching, urinary problems, impaired vision, confusion and restlessness or moodiness, particularly in some children.
Antihistamines are available in both over the counter and prescription forms. Some examples of brand name antihistamines that require a prescription include Atarax, Vistaril, Palgic, Xyzal and Clarinex. Non-prescription antihistamine drugs are classified into first-generation and second-generation antihistamines. While both types are effective for treating allergies, first-generation antihistamines are also used to manage motion sickness, states FamilyDoctor. Dimetapp is an example of a first-generation branded antihistamine, while Allegra falls under the category of a second-generation antihistamine.