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, dustparticles, certain foods, or medicines. These substances, known as allergens, cause no unusual reactions in most people. But in people who are sensitiveto 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 usedto temporarily relieve itching. 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. Check with the physician who prescribed the drug or the pharmacist who filled the prescription for the correct dosage, and always take antihistamines exactly as directed. Ifusing non-prescription (over-the-counter) types, follow the directions on thepackage label. Never take larger or more frequent doses, and do not take thedrug 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.

People who have seasonal allergies should take antihistamines before allergyseason starts or immediately after being exposed to an allergen. Even then, however, antihistamines do not cure allergies or prevent histamine from beingreleased. They can only be expected to reduce allergy symptoms by only about50%. In some people antihistamines become less effective when used over a long time. Switching to another type of antihistamine may help.

People with asthma, emphysema, chronic bronchitis, or other breathing problems should not use antihistamines unless directed to do so by a physician. Someantihistamines make people drowsy, dizzy, uncoordinated, or less alert. Forthis 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. Antihistamines can interfere with the results of skin and blood tests. Anyone who is taking antihistamines should notify the health care provider in charge before scheduling medical tests.

Because children are often more sensitive to antihistamines, they may be morelikely to have side effects and to suffer from accidental overdoses. Check with a physician before giving antihistamines to children under 12 years. Older people may also be more likely to have side effects, such as nervousness, irritability, dizziness, sleepiness, and low blood pressure from antihistamines. 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 all medical conditions. Anyone who has had unusual reactions to antihistamines in the past should let his or her physician know before taking the drugs again. The physicianshould 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 breastfeeding should check with their physiciansbefore using antihistamines.

Taking antihistamines with certain other drugs may affect the way the drugs work or may increase the chance of side effects. Antihistamines may increase the effects of other drugs that slow down the central nervous system (CNS), such as alcohol, tranquilizers, barbiturates, and sleep aids. Avoid drinking alcohol while taking antihistamines, and check with a physician before combining antihistamines with any other drugs. Common side effects of antihistaminesinclude drowsiness, dizziness, poor coordination, restlessness, excitability,nervousness, and upset stomach. These problems usually go away as the body adjusts to the drug and do not require medical treatment. Less common side effects, such as dry mouth, nose, and eyes, irritability, difficulty urinating,and blurred vision, also may occur and do not need medical attention unless they do not go away or they interfere with normal activities. Other rare sideeffects may occur. Anyone who has unusual symptoms after taking antihistamines should get in touch with his or her physician.

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