Antihistamines are used to relieve or prevent the symptoms of hay fever and other types of allergy. They work by preventing the effects of a substance called histamine, which is produced by the body. Histamine can cause itching, sneezing, runny nose, and watery eyes. Also, in some persons histamine can close up the bronchial tubes (air passages of the lungs) and make breathing difficult.

Some of the antihistamines are also used to prevent motion sickness, nausea, vomiting, and dizziness. In patients with Parkinson's disease, diphenhydramine may be used to decrease stiffness and tremors. Also, the syrup form of diphenhydramine is used to relieve the cough due to colds or hay fever. In addition, since antihistamines may cause drowsiness as a side effect, some of them may be used to help people go to sleep.

Hydroxyzine is used in the treatment of nervous and emotional conditions to help control anxiety. It can also be used to help control anxiety and produce sleep before surgery.

Some antihistamines are used in the treatment of chronic urticaria, which is a persistent hive-like rash.

Antihistamines may also be used for other conditions as determined by your doctor.

Antihistamine preparations are available both over-the-counter (OTC) and with your doctor's prescription.

Do not give any over-the-counter (OTC) cough and cold medicine to a baby or child under 4 years of age. Using these medicines in very young children might cause serious or possibly life-threatening side effects .

For patients taking this medicine by mouth:

Antihistamines can be taken with food or a glass of water or milk to lessen stomach irritation if necessary.
If you are taking the extended-release tablet form of this medicine, swallow the tablets whole. Do not break, crush, or chew before swallowing.
For patients taking dimenhydrinate or diphenhydramine for motion sickness:

Take this medicine at least 30 minutes or, even better, 1 to 2 hours before you begin to travel.
For patients using the suppository form of this medicine:

To insert suppository: First remove the foil wrapper and moisten the suppository with cold water. Lie down on your side and use your finger to push the suppository well up into the rectum. If the suppository is too soft to insert, chill the suppository in the refrigerator for 30 minutes or run cold water over it before removing the foil wrapper.
For patients using the injection form of this medicine:

If you will be giving yourself the injection, make sure you understand exactly how to give it. If you have any questions about this, check with your health care professional.
Antihistamines are used to relieve or prevent the symptoms of your medical problem. Take them only as directed. Do not take more of them and do not take them more often than recommended on the label, unless otherwise directed by your doctor. To do so may increase the chance of side effects.

Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur:

Less common or rare

Less common or rare with azatadine, cetirizine, clemastine, cyproheptadine, desloratadine, dexchlorpheniramine, diphenhydramine, or loratadine

Abdominal or stomach pain

burning

chills

clay-colored stools or dark urine

cough

diarrhea

difficulty swallowing

dizziness

fast or irregular heartbeat

fever

headache

hives

itching

prickly sensations

puffiness or swelling of the eyelids or around the eyes, face, lips or tongue

redness of skin

seizures

shortness of breath

skin rash

swelling

tightness in chest

tingling

unusual tiredness or weakness

wheezing

Check with your doctor as soon as possible if any of the following side effects occur:

Less common or rare

Sore throat

unusual bleeding or bruising

unusual tiredness or weakness

Symptoms of overdose

Clumsiness or unsteadiness

convulsions (seizures)

drowsiness (severe)

dryness of mouth, nose, or throat (severe)

feeling faint

flushing or redness of face

hallucinations (seeing, hearing, or feeling things that are not there)

shortness of breath or troubled breathing

trouble in sleeping

Some side effects may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects. Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

More common

Drowsiness

dry mouth, nose, or throat

gastrointestinal upset, stomach pain, or nausea

headache

increased appetite and weight gain

thickening of mucus

Less common or rare

Acid or sour stomach

belching

blurred vision or any change in vision

body aches or pain

clumsiness or unsteadiness

confusion (not with diphenhydramine)

congestion

constipation

cough

diarrhea

difficult or painful urination

difficulty in moving

difficult or painful menstruation

dizziness (not with brompheniramine or hydroxyzine)

drowsiness (with high doses of desloratadine and loratadine)

dryness of mouth, nose, or throat

early menstruation

fast heartbeat

fever

heartburn

hoarseness

increased sensitivity of skin to sun

increased sweating

indigestion

loss of appetite

joint pain

muscle aching or cramping

muscle pains or stiffness

nausea

nightmares (not with azatadine, chlorpheniramine, cyproheptadine, desloratadine, hydroxyzine, or loratadine)

ringing or buzzing in ears

runny nose

skin rash

swollen joints

stomach discomfort, upset or pain

tender swollen glands in neck

tremor

unusual excitement, nervousness, restlessness, or irritability

vomiting

Other side effects not listed may also occur in some patients. If you notice any other effects, check with your healthcare professional.

Call your doctor for medical advice about side effects. You may report side effects to the FDA at 1-800-FDA-1088.

Before you have any skin tests for allergies, tell the doctor in charge that you are taking this medicine. The results of the test may be affected by this medicine.

When taking antihistamines on a regular basis, make sure your doctor knows if you are taking large amounts of aspirin at the same time (as for arthritis or rheumatism). Effects of too much aspirin, such as ringing in the ears, may be covered up by the antihistamine.

Antihistamines will add to the effects of alcohol and other CNS depressants (medicines that slow down the nervous system, possibly causing drowsiness). Some examples of CNS depressants are sedatives, tranquilizers, or sleeping medicine; prescription pain medicine or narcotics; barbiturates; medicine for seizures; muscle relaxants; or anesthetics, including some dental anesthetics. Check with your doctor before taking any of the above while you are using this medicine.

This medicine may cause some people to become drowsy or less alert than they are normally. Even if taken at bedtime, it may cause some people to feel drowsy or less alert on arising. Some antihistamines are more likely to cause drowsiness than others. Drowsiness is less likely with cetirizine, and rare with desloratadine and loratadine. Make sure you know how you react to the antihistamine you are taking before you drive, use machines, or do anything else that could be dangerous if you are not alert.

Antihistamines may cause dryness of the mouth, nose, and throat. Some antihistamines are more likely to cause dryness of the mouth than others . For temporary relief of mouth dryness, use sugarless candy or gum, melt bits of ice in your mouth, or use a saliva substitute. However, if your mouth continues to feel dry for more than 2 weeks, check with your medical doctor or dentist. Continuing dryness of the mouth may increase the chance of dental disease, including tooth decay, gum disease, and fungus infections.

For patients using dimenhydrinate, diphenhydramine, or hydroxyzine:

This medicine controls nausea and vomiting. For this reason, it may cover up the signs of overdose caused by other medicines or the symptoms of appendicitis. This will make it difficult for your doctor to diagnose these conditions. Make sure your doctor knows that you are taking this medicine if you have other symptoms of appendicitis such as stomach or lower abdominal pain, cramping, or soreness. Also, if you think you may have taken an overdose of any medicine, tell your doctor that you are taking this medicine.
For patients using diphenhydramine or doxylamine as a sleeping aid:

If you are already taking a sedative or tranquilizer, do not take this medicine without consulting your doctor first.