Cetirizine (Aller-Tec, Alleroff, Zyrtec, Reactin, cetirizine hydrochloride) belongs to a class of drugs called antihistamines. Antihistamines work by blocking the action of histamine on your cells. Your body produces histamines when it is exposed to allergens, such as pollen, pet dander and house dust. Histamines attach to cells in your body causing those cells to swell and leak fluid, and this is what leads to the classic allergy symptoms of itchy, watery eyes, sneezing, a runny nose and an itchy, sore throat. Antihistamines block these effects. They don’t prevent allergies, and they don’t treat the underlying pathology. Cetirizine is a second generation antihistamine used to treat allergic symptoms and, in some cases, rashes and redness (urticaria) caused by hives. It is also used to treat angioedema, which is the sudden and rapid swelling of the tissues under the skin. Cetirizine is available over-the-counter as tablets, chewable tablets, extended release tablets and syrup. You can also buy it in combination with pseudoephedrine in products such as Sudafed. If you have hives that are bruised or blistered, are an unusual color, or that do not itch, call your doctor before taking this or any medication. If you have pre-existing medical conditions including asthma, thyroid disease or high blood pressure, you should speak with your doctor before using cetirizine. Side effects associated with cetirizine include sleepiness, dry mouth, nausea, headache, fatigue, jitteriness and sore throat. Serious adverse effects include difficulty breathing or swallowing: if you experience these symptoms contact your physician immediately.