Antihistamines

Antihistamines are medicines that relieve or prevent the symptoms of hay fever and other kinds of allergies. 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 familiar and annoying allergy symptoms such as sneezing, itching, runny nose, and watery eyes.

As their name suggests, antihistamines block the effects of histamine, and so reduce 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.

By blocking the action of histamine, antihistamines prevent symptoms that include headaches, irregular heart beat, and impaired breathing due to contracted bronchial tubes. In those with severe allergies, the use of antihistamines can be life-saving.

However, the benefit of antihistamines comes with side effects that make their use cautionary. 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).

Recommended dosage depends on the type of antihistamine. It is unwise to take larger or more frequent doses, or to take the drug longer than directed. For best effects, take antihistamines on a schedule, not just as needed. Histamine is released more or less continuously, so countering its effects requires regular use of antihistamines.

Those with seasonal allergies should take antihistamines before allergy season starts or immediately after being exposed to an allergen. Even then, however, antihistamines do not cure allergies or prevent histamine from being released. They also have no effect on other chemicals that the body releases when exposed to allergens. For these reasons, antihistamines can be expected to reduce allergy symptoms by only about 50%. For some people antihistamines become less effective when used over a long time. Switching to another type of antihistamine may help.

The antihistamines Seldane and Seldane-D were taken off the market in 1997 due to concerns that the active ingredient, terfenadine, triggered life-threatening heart rhythm problems when taken with certain drugs. Terfenadine can also be dangerous to people with liver disease. The U.S. Food and Drug Administration encouraged people who were using Seldane to talk to their physicians about switching to another antihistamine such as loratadine (Claritin) or to fexofenadine (Allegra), which is similar to Seldane but has a safer active ingredient.

Astemizole (Hismanal) may also cause life-threatening heart rhythm problems or severe allergic reactions when taken in higher-than-recommended doses

KEY TERMS

Allergen— An otherwise harmless substance that can cause a hypersensitive allergic response.

Anaphylaxis— A sudden, life-threatening allergic reaction.

Hallucination— A sensory experience of something that does not exist outside the mind. A person can experience a hallucination in any of the five senses. Hallucinations usually result from drugs or mental disorders.

Histamine— A chemical released from cells in the immune system as part of an allergic reaction.

Phenylketonuria— (PKU) A genetic disorder in which the body lacks an important enzyme. If untreated, the disorder can lead to brain damage and mental retardation.

Pollen— Dust-like grains produced by the male parts of plants and carried by wind, insects, or other methods, to the female parts of plants.

or with certain drugs. The drug was withdrawn from the U.S. market in 1999.

People with asthma, emphysema, chronic bronchitis, sleep apnea, or other breathing problems should not use antihistamines unless directed to do so by a physician.

Some antihistamines make people drowsy, dizzy, uncoordinated, or less alert. For this reason, anyone who takes these drugs should not drive, use machines, or do anything else that might be dangerous until they have found out how the drugs affect them.

Nancy Ross-Flanigan