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Recommended Dosage, Precautions, Side Effects, InteractionsSpecial conditions

Antihistamines are medicines that relieve or prevent the symptoms of hay fever and other kinds of allergy. An allergy is a condition in which the body becomes unusually sensitive to some substance, such as pollen, mold spores, dust particles, certain foods, or medicines. These substances, known as allergens, cause no unusual reactions in most people. But in people who are sensitive to them, exposure to allergens causes the immune system to overreact. The main reaction is the release of a chemical called histamine from specialized cells in the body tissues. Histamine causes such familiar and annoying allergy symptoms as sneezing, itching, runny nose, and watery eyes.

As their name suggests, antihistamines block the effects of histamine, reducing allergy symptoms. When used for this purpose, they work best when taken before symptoms are too severe. Antihistamine creams and ointments may be used to temporarily relieve itching. Some antihistamines are also used to treat motion sickness, nausea, dizziness, and vomiting. And because some cause drowsiness, they may be used as sleep aids.

Some antihistamine products are available only with a physician's prescription. Others can be bought without a prescription. These drugs come in many forms, including tablets, capsules, liquids, injections and suppositories. Some common antihistamines are astemizole (Hismanal), brompheniramine (Dimetane, Dimetapp), chlorpheniramine (Deconamine), clemastine (Tavist), diphenhydramine (Benadryl), doxylamine (an ingredient in sleep aids such as Unisom and Vicks NyQuil), loratadine (Claritin), and promethazine (Phenergan).

People with certain medical conditions or who are taking certain other medicines can have problems if they take antihistamines. Before taking these drugs, be sure to let the physician or pharmacist know about any of these conditions:


Some antihistamine products may contain the dye tartrazine, which causes allergic reactions, including bronchial asthma, in some people. People who are allergic to aspirin may also be allergic to tartrazine.

Sodium bisulfite, a preservative, is found in some antihistamine products. People who are sensitive to this chemical may have allergic-type reactions, including anaphylaxis and severe asthma attacks. People with asthma are especially likely to be sensitive to sodium bisulfite. Ask the pharmacist which antihistamine products are sulfite-free.

Anyone who has had unusual reactions to antihistamines in the past should let his or her physician know before taking the drugs again. The physician should also be told about any allergies to foods, dyes, preservatives, or other substances.


Pregnant women should not use antihistamines unless directed to do so by a physician.


Antihistamines pass into breast milk and may cause side effects in nursing babies. Women who are breast-feeding should check with their physicians before using antihistamines.

Other medical conditions

Before using antihistamines, people with any of these medical problems should make sure their physicians are aware of their conditions:

  • glaucoma
  • hyperthyroidism (overactive thyroid)
  • high blood pressure
  • enlarged prostate
  • heart disease
  • ulcers or other stomach problems
  • stomach or intestinal blockage
  • liver disease
  • kidney disease
  • bladder obstruction
  • diabetes

Use of certain medicines

Taking antihistamines with certain other drugs may affect the way the drugs work or may increase the chance of side effects.

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