Most common allergy symptoms—sneezing, itchy eyes, runny nose—are caused by chemicals called histamines, which the body produces to fight off allergens. Drugs that can counteract these chemicals and relieve symptoms are called antihistamines.

Antihistamines are the most commonly used medications to reduce symptoms caused by pollen, ragweed, dust mites, mold, animal dander, and cockroaches, to name a few. If a person is allergic to any of these triggers, the body produces histamines to fight off the allergens by attaching to cells called histamine receptors, causing inflammation and mucus production. Antihistamines can bond to these receptors instead, preventing histamines from causing symptoms.

Some antihistamines are sold over-the-counter, while others require a prescription. They can be taken as tablets, capsules, or liquids, or, less frequently, by injection or suppository. The most common side effect of antihistamines is drowsiness—it’s so common, in fact, that the drug is also used in nighttime cough and cold formulas and over-the-counter sleep medications. Other possible side effects, especially in older people with other health conditions, may include upset stomach, constipation, headache, and dry mouth. These medications should not be given to anyone under the age of 4, as side effects for young children can be life threatening.

Over-the-counter antihistamines include brompheniramine, diphenhydramine, and loratadine, among others. These are the active ingredients in the Dimetapp, Benadryl, and Claritin brand products, respectively. They are often mixed with other drugs, such as pain relievers or decongestants, to treat many symptoms of a cold, the flu, or allergies at once. Prescription antihistamines include fexofenadine (Allegra), cetirizine (Zyrtec), and desloratadine (Clarinex). Prescription nasal sprays such as azelastine (Astelin) are also available. Studies have found these medications to be no more effective than the over-the-counter versions, although they are much less likely to cause drowsiness.

Because antihistamines can cause drowsiness, people taking the drugs are warned to be especially careful while driving a car or operating machinery. Sleeping pills, sedatives, muscle relaxants, blood pressure medications, and alcohol can increase drowsiness caused by antihistamines.

ADDITIONAL FACTS

  1. People with asthma can have more-severe allergic symptoms because their airways are more sensitive. Antihistamines may help keep the airways from closing up during an allergic reaction.
  2. Antihistamines are sometimes used to treat a chronic hivelike rash called urticaria; they’re also prescribed to combat nausea, headaches, anxiety, stiffness, and tremors in patients with Parkinson’s disease.
  3. Antihistamines can also relieve itchiness caused by insect bites and stings, poison ivy, and poison oak.