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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.

Dosing

The dose of an antihistamine will be different for different patients. Follow your doctor's orders or the directions on the label. The following information includes only the average doses of antihistamines. If your dose is different, do not change it unless your doctor tells you to do so.

The number of capsules or tablets or teaspoonfuls of liquid that you take or the number of suppositories you use depends on the strength of the medicine. Also, the number of doses you take each day and the time between doses depends on whether you are taking a short-acting or long-acting form of antihistamine.

• For use as an antihistamine:
  • For azatadine
    • For oral dosage form (tablets):
      • Adults: 1 to 2 milligrams (mg) every eight to twelve hours as needed.
      • Children younger than 12 years of age: Use and dose must be determined by your doctor.
      • Children 12 years of age and older: 0.5 mg to 1 mg two times a day as needed.
  • For brompheniramine
    • For regular (short-acting) oral dosage forms (capsules, tablets, or liquid):
      • Adults and teenagers: 4 milligrams (mg) every four to six hours as needed.
      • Children 2 to 6 years of age: 1 mg every four to six hours as needed.
      • Children 6 to 12 years of age: 2 mg every four to six hours as needed.
    • For injection dosage form:
      • Adults and teenagers: 10 milligrams (mg) injected into a muscle, under the skin, or into a vein every eight to twelve hours.
      • Children younger than 12 years of age: 0.125 mg per kilogram (0.06 mg per pound) of body weight injected into a muscle, under the skin, or into a vein three or four times a day as needed.
  • For cetirizine
    • For oral dosage forms (syrup and tablets):
      • Adults: 5 to 10 milligrams (mg) once a day.
      • Children younger than 2 years of age: Use and dose must be determined by your doctor.
      • Children 2 to 6 years of age: 2.5 mg once a day, up to a maximum of 5 mg once a day or 2.5 mg twice a day.
      • Children 6 years of age and older: 5 to 10 mg once a day.
  • For chlorpheniramine
    • For regular (short-acting) oral dosage forms (tablets or liquid):
      • Adults and teenagers: 4 milligrams (mg) every four to six hours as needed.
      • Children younger than 6 years of age: Use and dose must be determined by your doctor.
      • Children 6 to 12 years of age: 2 mg three or four times a day as needed.
    • For long-acting oral dosage forms (capsules or tablets):
      • Adults: 8 or 12 milligrams (mg) every eight to twelve hours as needed.
      • Children younger than 12 years of age: Use and dose must be determined by your doctor.
      • Children 12 years of age and older: 8 mg every twelve hours as needed.
    • For injection dosage form:
      • Adults: 5 to 40 milligrams (mg) injected into a muscle, into a vein, or under the skin.
      • Children: 0.0875 mg per kilogram (0.04 mg per pound) of body weight injected under the skin every six hours as needed.
  • For clemastine
    • For oral dosage forms (tablets or liquid):
      • Adults and teenagers: 1.34 milligrams (mg) two times a day or 2.68 mg one to three times a day as needed.
      • Children younger than 6 years of age: Use and dose must be determined by your doctor.
      • Children 6 to 12 years of age: 0.67 to 1.34 mg two times a day.
  • For cyproheptadine
    • For oral dosage forms (tablets or liquid):
      • Adults and children 14 years of age and older: 4 milligrams (mg) every eight hours. The doctor may increase the dose if needed.
      • Children 2 to 6 years of age: 2 mg every eight to twelve hours as needed.
      • Children 6 to 14 years of age: 4 mg every eight to twelve hours as needed.
  • For desloratadine
    • For oral dosage form (tablets):
      • Adults and children 12 years of age and older: 5 milligrams (mg) once a day.
      • Children younger than 12 years of age: Use and dose must be determined by your doctor.
  • For dexchlorpheniramine
    • For regular (short-acting) oral dosage forms (tablets or liquid):
      • Adults and teenagers: 2 milligrams (mg) every four to six hours as needed.
      • Children 2 to 5 years of age: 0.5 mg every four to six hours as needed.
      • Children 5 to 12 years of age: 1 mg every four to six hours as needed.
    • For long-acting oral dosage form (tablets):
      • Adults: 4 or 6 milligrams (mg) every eight to twelve hours as needed.
      • Children: Use and dose must be determined by your doctor.
  • For diphenhydramine
    • For oral dosage forms (capsules, tablets, or liquid):
      • Adults and teenagers: 25 to 50 milligrams (mg) every four to six hours as needed.
      • Children younger than 6 years of age: 6.25 to 12.5 mg every four to six hours.
      • Children 6 to 12 years of age: 12.5 to 25 mg every four to six hours.
    • For injection dosage form:
      • Adults: 10 to 50 milligrams (mg) injected into a muscle or into a vein.
      • Children: 1.25 mg per kg (0.6 mg per pound) of body weight injected into a muscle four times a day.
  • For doxylamine
    • For oral dosage form (tablets):
      • Adults and teenagers: 12.5 to 25 milligrams (mg) every four to six hours as needed.
      • Children younger than 6 years of age: Use and dose must be determined by your doctor.
      • Children 6 to 12 years of age: 6.25 to 12.5 mg every four to six hours as needed.
  • For fexofenadine
    • For oral dosage forms (capsules):
      • Adults and teenagers: 60 milligrams (mg) two times a day as needed or 180 mg once a day
      • Children 6 to 11 years of age: 30 mg twice a day as needed.
      • Children under 6 years of age: Use and dose must be determined by your doctor.
  • For loratadine
    • For oral dosage forms (tablets or liquid):
      • Adults and children 6 years of age and older: 10 milligrams (mg) once a day.
      • Children 2 to 5 years of age: 5 mg once a day.
  • For phenindamine
    • For oral dosage form (tablets):
      • Adults and teenagers: 25 milligrams (mg) every four to six hours as needed.
      • Children younger than 6 years of age: Use and dose must be determined by your doctor.
      • Children 6 to 12 years of age: 12.5 mg every four to six hours as needed.
• For nausea, vomiting, and vertigo (only dimenhydrinate and diphenhydramine are used for vertigo):
  • For dimenhydrinate
    • For regular (short-acting) oral dosage forms (tablets or liquid):
      • Adults and teenagers: 50 to 100 milligrams (mg) every four to six hours as needed.
      • Children 2 to 6 years of age: 12.5 to 25 mg every six to eight hours as needed.
      • Children 6 to 12 years of age: 25 to 50 mg every six to eight hours as needed.
    • For long-acting oral dosage form (capsules):
      • Adults: 1 capsule (contains 25 milligrams [mg] for immediate action and 50 mg for long action) every twelve hours.
      • Children: Use and dose must be determined by your doctor.
    • For injection dosage form:
      • Adults: 50 milligrams (mg) injected into a muscle or into a vein every four hours as needed.
      • Children: 1.25 mg per kg (0.6 mg per pound) of body weight injected into a muscle or into a vein every six hours as needed.
    • For suppository dosage form:
      • Adults: 50 to 100 milligrams (mg) inserted into the rectum every six to eight hours as needed.
      • Children younger than 6 years of age: Use and dose must be determined by your doctor.
      • Children 6 to 8 years of age: 12.5 to 25 mg inserted into the rectum every eight to twelve hours as needed.
      • Children 8 to 12 years of age: 25 to 50 mg inserted into the rectum every eight to twelve hours as needed.
      • Children 12 years of age and older: 50 mg inserted into the rectum every eight to twelve hours as needed.
  • For diphenhydramine
    • For oral dosage forms (capsules, tablets, or liquid):
      • Adults: 25 to 50 milligrams (mg) every four to six hours as needed.
      • Children: 1 to 1.5 mg per kg (0.45 to 0.7 mg per pound) of body weight every four to six hours as needed.
    • For injection dosage form:
      • Adults: 10 milligrams (mg) injected into a muscle or into a vein. Dose may be increased to 25 to 50 mg every two to three hours.
      • Children: 1 to 1.5 mg per kg (0.45 to 0.68 mg per pound) of body weight injected into a muscle every six hours.
  • For hydroxyzine
    • For oral dosage forms (capsules, tablets, or liquid):
      • Adults: 25 to 100 milligrams (mg) three or four times a day as needed.
      • Children younger than 6 years of age: 12.5 mg every six hours as needed.
      • Children 6 years of age and older: 12.5 to 25 mg every six hours as needed.
      For injection dosage form:
      • Adults: 25 to 100 milligrams (mg) injected into a muscle.
      • Children: 1 mg per kg (0.45 mg per pound) of body weight injected into a muscle.
• For Parkinson's disease:
  • For diphenhydramine
    • For oral dosage forms (capsules, tablets, or liquid):
      • Adults: 25 milligrams (mg) three times a day when starting treatment. Your doctor may increase the dose gradually later if needed.
    • For injection dosage form:
      • Adults: 10 to 50 milligrams (mg) injected into a muscle or into a vein.
      • Children: 1.25 mg per kg (0.6 mg per pound) of body weight four times a day injected into a muscle.
• For use as a sedative (to help sleep):
  • For diphenhydramine
    • For oral dosage forms (capsules, tablets, or liquid):
      • Adults: 50 milligrams (mg) twenty to thirty minutes before bedtime if needed.
  • For doxylamine
    • For oral dosage form (tablets):
      • Adults: 25 milligrams (mg) thirty minutes before bedtime if needed.
      • Children: Use and dose must be determined by your doctor.
  • For hydroxyzine
    • For oral dosage forms (capsules, tablets, or liquid):
      • Adults: 50 to 100 milligrams (mg).
      • Children: 0.6 mg per kg (0.3 mg per pound) of body weight.
    • For injection dosage form:
      • Adults: 50 milligrams (mg) injected into a muscle.
• For anxiety:
  • For hydroxyzine
    • For oral dosage forms (capsules, tablets, or liquid):
      • Adults: 50 to 100 milligrams (mg).
      • Children: 0.6 mg per kilogram (0.3 mg per pound) of body weight.
    • For injection dosage form:
      • Adults: 50 to 100 milligrams (mg) injected into a muscle every four to six hours as needed.
      • Children: 1 mg per kilogram (0.45 mg per pound) of body weight injected into a muscle.

Missed dose

If you are taking this medicine regularly and you miss a dose, take it as soon as possible. However, if it is almost time for your next dose, skip the missed dose and go back to your regular dosing schedule. Do not double doses.

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 injections, make sure you understand exactly how to give them. If you have any questions about this, check with your health care professional.

Storage

To store this medicine:

• Keep out of the reach of children, since overdose may be very dangerous in children.
• Store away from heat and direct light.
• Do not store the capsule or tablet form of this medicine in the bathroom medicine cabinet, near the kitchen sink, or in other damp places. Heat or moisture may cause the medicine to break down.
• Keep the liquid form of this medicine from freezing.
• Do not keep outdated medicine or medicine no longer needed. Be sure that any discarded medicine is out of the reach of children.

In deciding to use a medicine, the risks of taking the medicine must be weighed against the good it will do. This is a decision you and your doctor will make. For antihistamines, the following should be considered:

Allergies Tell your doctor if you have ever had any unusual or allergic reaction to antihistamines. Also tell your health care professional if you are allergic to any other substances, such as foods, preservatives, or dyes.

Diet Make certain your health care professional knows if you are on a low-sodium, low-sugar, or any other special diet. Most medicines contain more than their active ingredient, and many liquid medicines contain alcohol.

Pregnancy Hydroxyzine is not recommended for use in the first months of pregnancy since it has been shown to cause birth defects in animal studies when given in doses many times higher than the usual human dose. Be sure you have discussed this with your doctor.

Desloratadine and fexofenadine have not been studied in pregnant women. However, studies in animals have shown that these medicines cause birth defects or other problems when given in doses higher than the usual human dose. Before taking this medicine, make sure your doctor knows if you are pregnant or if you may become pregnant.

Azatadine, brompheniramine, cetirizine, chlorpheniramine, clemastine, cyproheptadine, dexchlorpheniramine, dimenhydrinate, diphenhydramine, doxylamine, and loratadine have not been studied in pregnant women. However, these medicines have not been shown to cause birth defects or other problems in animal studies.

Breast-feeding Small amounts of antihistamines pass into the breast milk. Use is not recommended since babies are more susceptible to the side effects of antihistamines, such as unusual excitement or irritability. Also, since these medicines tend to decrease the secretions of the body, it is possible that the flow of breast milk may be reduced in some patients. It is not known yet whether cetirizine, desloratadine, or loratadine cause these same side effects.

Children Serious side effects, such as convulsions (seizures), are more likely to occur in younger patients and would be of greater risk to infants than to older children or adults. In general, children are more sensitive to the effects of antihistamines. Also, nightmares or unusual excitement, nervousness, restlessness, or irritability may be more likely to occur in children.

Older adults Elderly patients are usually more sensitive to the effects of antihistamines. Confusion; difficult or painful urination; dizziness; drowsiness; feeling faint; or dryness of mouth, nose, or throat may be more likely to occur in elderly patients. Also, nightmares or unusual excitement, nervousness, restlessness, or irritability may be more likely to occur in elderly patients.

Other medicines Although certain medicines should not be used together at all, in other cases different medicines may be used together even if an interaction might occur. In these cases, your doctor may want to change the dose, or other precautions may be necessary. When you are taking antihistamines it is especially important that your health care professional knows if you are taking any of the following:

• Anticholinergics (medicine for abdominal or stomach spasms or cramps) Side effects, such as dryness of mouth, of antihistamines or anticholinergics may be more likely to occur

• Erythromycin (e.g., E-Mycin) or
• Ketoconazole (e.g., Nizoral) Use of these medicines with fexofenadine may cause an increased amount of fexofenadine in the blood.

• Central nervous system (CNS) depressants (medicines that cause drowsiness) Effects, such as drowsiness, of CNS depressants or antihistamines may be worsened; also, taking maprotiline or tricyclic antidepressants may cause some side effects of either of these medicines, such as dryness of mouth, to become more severe

• Monoamine oxidase (MAO) inhibitor activity (isocarboxazid [e.g., Marplan], isocarboxazid [e.g., Marplan], phenelzine [e.g., Nardil], procarbazine [e.g., Matulane], selegiline [e.g., Eldepryl], tranylcypromine [e.g., Parnate]) If you are now taking, or have taken within the past 2 weeks, any of the MAO inhibitors, the side effects of the antihistamines, such as drowsiness and dryness of mouth, may become more severe; these medicines should not be used together

• Enlarged prostate or
• Urinary tract blockage or difficult urination Antihistamines may make urinary problems worse

• Glaucoma These medicines may cause a slight increase in inner eye pressure that may make the condition worse

• Intestinal obstruction or
• Stomach ulcer Use of cyproheptadine may make these conditions worse.

• Liver disease or
• Kidney disease Effects of desloratadine may be increased because of slower removal from the body.