Antihistamines (Oral) (cont.)

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What are the side effects of oral antihistamines?

First generation antihistamines are used less often to treat allergies because they cause significant sedation. First generation antihistamines also should be used cautiously in older adults as they are more susceptible to their anticholingeric side effects including

Due to their significant side effect profile, special precautions should be used in patients with:

Second generation antihistamines are less sedating than their first generation counterparts. Cetirizine can be sedating for some patients at normal recommended doses while sedation seems to only be a concern with loratadine at higher than normally recommended doses. Fexofenadine is the least sedating.

Side effects common to all antihistamines include:

  • Dizziness
  • Dry mouth
  • Dry eyes
  • Blurred vision
  • Problems urinating
  • Constipation
  • Mental disturbances

What drugs interact with oral antihistamines?

Antihistamines may cause drowsiness. Taking antihistamines with other medicines that also are sedating may cause profound drowsiness. Examples include:

  • Sleeping pills
  • Alcohol
  • Seizure medications
  • Some antidepressants (especially tricyclic antidepressants)
  • Antipsychotics
  • Muscle relaxants
  • Prescription narcotic pain medications
  • Other antihistamines
  • MAOIs (monoamine oxidase inhibitors), a class of medications used to treat mood disorders, may prolong or intensify the anticholinergic side effects of antihistamines. Generally, first generation antihistamines should not be used within two weeks of using a MAOI.
Medically Reviewed by a Doctor on 7/9/2015


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