How to Take Your Medicine:  Antihistamine 

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
hay 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