Clemastine

Definition

Antihistamines are medicines that relieve or prevent the symptoms of hay fever and other kinds of allergy.

Purpose

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.

Description

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

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. If using non-prescription (over-the-counter) types, follow the directions on the package label. Never take larger or more frequent doses, and do not 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.

Precautions

For best effects, people who have 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%.

In 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.

The antihistamine astemizole (Hismanal) may also cause life-threatening heart rhythm problems or severe allergic reactions when taken in higher-than-recommended doses or with certain drugs. Taking astemizole with food may reduce its absorption into the bloodstream. Astemizole (Hismanal) should not be combined with any of the following:

• The antibiotics erythromycin (E-Mycin and other brands), clarithromycin (Biaxin), or troleandomycin (TAO)
• The blood pressure medicine mibefradil (Posicor)
• Medicines used in treating HIV infection such as indinavir (Crixivan), ritonavir (Norvir), and nelfinavir (Viracept)
• Antidepressants such as fluoxetine (Prozac), sertraline (Zoloft), or paroxetine (Paxil)
• Asthma medicines such as zileuton (Zyflo)
• The antifungal drugs ketoconazole (Nizoral) or itraconazole (Sporanox)
• Large doses of quinine.

In addition, patients with liver disease should not take Hismanal.

People with asthma, emphysema, chronic bronchitis, 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.

Antihistamines can interfere with the results of skin and blood tests. The antihistamine promethazine (Phenergan) can interfere with pregnancy tests and can raise blood sugar. Anyone who is taking antihistamines should notify the health care provider in charge before scheduling medical tests.

People with phenylketonuria should be awarethat some antihistamine products contain aspartame (Nutrasweet), which breaks down in the body to phenylalanine.

Anyone who has sleep apnea (periods when breathing stops during sleep) should not take the antihistamine promethazine (Phenergan). This drug may also cause people with seizure disorders to have more frequent seizures.

Because children are often more sensitive to antihistamines, they may be more likely 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. Older men may also have problems urinating. Unless these problems are severe, they can usually be handled by taking a lower dose or switching to a different antihistamine. However, people over 80 or older people with serious physical problems or dementia may become confused, disoriented, and incoherent after taking even small amounts of the antihistamine diphenhydramine (Benadryl).

Special conditions

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:

Allergies

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 breastfeeding should check with their physicians before using antihistamines.

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.

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

Side effects

Common side effects of antihistamines include 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.

If any of the following side effects occur, check with the physician who prescribed the medicine as soon as possible:

• Rapid, irregular, pounding, or fluttering heartbeat
• Convulsions
• Sweating
• Vomiting
• Fever
• Chills
• Fainting
• Breathing problems
• Hallucinations
• Unusual bleeding or bruising
• Low blood pressure
• Unusual sensitivity to light
• Uncontrolled movements.

Other rare side effects may occur. Anyone who has unusual symptoms after taking antihistamines should get in touch with his or her physician.

Interactions

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 other CNS depressants.

Certain antihistamines should not be used within two weeks of using monoamine oxidase inhibitors (MAO inhibitors), which are drugs used to treat Parkinson's disease, depression, and other psychiatric conditions. Examples of MAO inhibitors are Parnate and Nardil. People who have been taking MAO inhibitors or who are not sure if they have should check with their physician or pharmacist before taking antihistamines.

Although no such interactions have been reported, antihistamine loratadine (Claritin) could potentially interact with the antiulcer drugs cimetidine (Tagamet) and ranitidine (Zantac), both of which are also taken for heartburn; with antibiotics such as erythromycin and Biaxin; with the antifungal drug ketoconazole (Nizoral); and with the bronchodilator theophylline (Theo-Dur). These interactions can cause liver problems. Persons who are taking these drugs should consult with an physician or pharmacist to see whether this is an issue for them.

Check with a physician before combining the antihistamine chlorpheniramine (Deconamine) with any of the following:

• Asthma medicines such as albuterol (Proventil) and bromocriptine (Parlodel)
• Blood pressure drugs such as mecamylamine (Inversine), methyldopa (Aldomet), and reserpine
• Narcotic painkillers such as meperidine (Demerol), oxycodone-aspirin (Percodan) and oxycodone-acetaminophen (Percocet)
• The anticonvulsant drug phenytoin (Dilantin)
• Sleep aids such as triazolam (Halcion) and secobarbital (Seconal)
• Tranquilizers such as diazepam (Valium) and alprazolam (Xanax).

Hismanal should not be taken with grapefruit juice or combined with any of the following drugs:

• Antibiotics such as erythromycin (E-Mycin and other brands), clarithromycin (Biaxin), or troleandomycin (TAO)
• The blood pressure medicine mibefradil (Posicor)
• Medicines used in treating HIV infection such as indinavir (Crixivan), ritonavir (Norvir), and nelfinavir (Viracept)
• Antidepressants such as fluoxetine (Prozac), sertraline (Zoloft), or paroxetine (Paxil)
• Asthma medicines such as zileuton (Zyflo)
• The antifungal drugs ketoconazole (Nizoral) or itraconazole (Sporanox)
• Large doses of quinine.

Check with a physician before combining any antihistamine with any of the following drugs:

• Medicines for stomach or abdominal cramps or spasms, such as dicyclomine (Bentyl)
• The antibiotics azithromycin (Zithromax), clarithromycin (Biaxin), or erythromycin (E-mycin)
• The antifungal drugs Itraconazole (Sporanox) or Ketoconazole (Nizoral)
• The calcium channel blocker bepridil (Vascor)
• Drugs used to treat irregular heartbeat, such as disopyramide (Norpace), procainamide (Pronestyl), or quinidine (Quinaglute Dura-Tabs, Cardioquin)
• Antidepressants such as maprotiline (Ludiomil) or tricyclic antidepressants such as desipramine (Norpramin) or imipramine (Tofranil)
• Medicines called phenothiazines, used to treat mental, emotional, and nervous disorders. Examples are chlorpromazine (Thorazine) and prochlorperazine (Compazine).
• Pimozide (Orap), used to treat symptoms of Tourette's syndrome.

Not all possible interactions of antihistamines with other drugs are listed here. Be sure to check with a physician or pharmacist before combining antihistamines with any other prescription or nonprescription (over-the-counter) medicine.

Key Terms

Allergen

A substance that causes an allergy.

Anaphylaxis

A sudden, life-threatening allergic reaction.

Hallucination

A false or distorted perception of objects, sounds, or events that seems real. 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.

Further Reading

For Your Information

Organizations

• Allergy and Asthma Network. 3554 Chain Bridge Road, Suite 200. 800-878-4403.
• American Academy of Allergy and Immunology. 611 East Wells Street, Milwaukee, WI 53202. 800-822-2762
• Asthma and Allergy Foundation of America. 1125 15th Street NW, Suite 502, Washington, DC 20005. 800-727-8462

Gale Encyclopedia of Medicine. Gale Research, 1999.