Antihistamine

by Food and Drug Administration (FDA)

How you take a medication makes a big difference in how well it works and how safe it will be for you. Sometimes it can be as important as what you take. Timing, what you eat and when you eat, proper dose, and many other factors can mean the difference between getting better, staying the same, or even getting worse. This drug information page is intended to help you make your medication work as effectively as possible. It is important to note, however, that this is only a guideline. You should talk to your doctor or pharmacist about how and when to take any prescribed drug.

This installment in a series of articles on commonly prescribed medications is about antihistamines. Some of these medications are available over-the-counter, or by prescription.

Antihistamines work by temporarily blocking the action of a substance produced by the body called histamine, which can cause itching, sneezing, runny nose and eyes, and other symptoms. Most of these drugs can have a drying effect on the nasal mucous. They are structurally related to local anesthetics and can produce sedation. The reason why some antihistamines are effective against motion sickness and some symptoms of Parkinson's disease is not known.

Conditions These Drugs Treat

Hight fever and other allergies

A few antihistamines have special uses, such as for treating:

cough due to colds or inhaled irritants
motion sickness
sleeplessness
hives
stiffness and tremors in patients with Parkinson's disease

How to Take

Extended-release tablets or capsules should be swallowed whole. The patient should follow directions on the container (or doctor's directions) on how often to take them.

Astemizole, one of the new relatively non-sedating antihistamines, is not absorbed well unless it is taken on an empty stomach, with no food one hour before and two hours after the medication.

Dimenhydrinate and diphenhydramine for motion sickness should be taken at least 30 minutes before travel and are most effective when taken one to two hours beforehand.

Missed Doses

These drugs are for symptomatic relief, and a missed dose does not have any harmful consequences except for possible return of the symptoms. Too frequent dosing may cause increased sedation and other side effects.

Side Effects and Risks

Common side effects such as drowsiness may decrease somewhat as your body adjusts to the medicine.

Notify your doctor as soon as possible if you notice any of these symptoms after taking antihistamines:

blurred vision
painful or difficult urination
unusual tiredness or severe drowsiness
weakness, clumsiness or unsteadiness
marked dryness of the mouth, nose or throat
fainting, seizures, or other loss of consciousness
hallucinations
shortness of breath.

Some of these may be due to overdose, others to individual intolerance to the medication.

Precautions and Warnings

Most antihistamines cause some people to become drowsy. Make sure you know how you react to the drug you are taking before you drive or operate machinery. The elderly may be particularly susceptible to the sedative effect.

Do not give antihistamines to children under 6 years without consulting a doctor.

Antihistamines add to the effects of alcohol and other depressants. Therefore, avoid taking them close to the same time you drink alcohol or use drugs that slow down the nervous system, such as sedatives, tranquilizers, sleeping pills, narcotics, prescription pain medicines, seizure medications, muscle relaxants, or anesthetics.

Antihistamines may also mask nausea associated with overdose from other medicines you are taking or from other medical conditions. If you suspect such effects while taking antihistamines, be sure to inform your doctor.

When buying over-the-counter products, be sure to read the ingredient labels to make certain you are not taking more than one product containing antihistamines. If you are already taking a sedative or tranquilizer, do not take antihistamines, including those sold over-the-counter as sleep aids, without checking with your doctor.

If you get skin tests for allergies or are receiving allergy injections, tell your doctor if you are taking antihistamines because these drugs can distort test results and mask reactions.

For relief of dry mouth, nose and throat caused by antihistamines, use sugarless candy or gum or melt pieces of ice in your mouth. If dryness continues for two weeks or more, it may increase the chance of dental disease. Check with your doctor or dentist if the symptom persists.

Generic Name

astemizole
azatadine
bromodiphenhydramine
* brompheniramine
carbinoxamine
* chlorpheniramine
clemastine
cyproheptadine
* dexbrompheniramine
* dexchlorpheniramine
* dimenhydrinate
* diphenhydramine
diphenylpyraline
* doxylamine
phenindamine
* pyrilamine
terfenadine
tripelennamine
*triprolidine

About the Author

www.fda.gov
FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.