Antihistamines

Many antihistamine drugs are available without a prescription. Examples include the first-generation antihistamines such as brompheniramine (Dimetapp, Bromphen, Dimetane, Nasahist), chlorpheniramine (Chlor-Trimeton), clemastine (Allerhist, Tavist), and diphenhydramine (Benadryl), and a second-generation antihistamine loratadine (Claritin). Loratadine (Claritin) does not cause drowsiness.

Common prescription antihistamines (all second-generation antihistamines) include cetirizine (Zyrtec), desloratadine (Clarinex), and fexofenadine (Allegra). These antihistamines (and loratadine) are less likely to cause adverse effects like drowsiness or dry mouth.

Several antihistamine nasal sprays (for example, azelastine [Astelin]) are also available to treat symptoms such as runny nose, sneezing, and itchy nose.

• How antihistamines work: These drugs compete with histamine for histamine receptor sites. By occupying the histamine receptor sites, they prevent histamine from causing allergic symptoms. Antihistamines are most effective when taken continuously during the allergy season.



• Who should not use these medications: Antihistamines are contraindicated in individuals allergic to them. They may cause unwanted side effects in the following:
  
  
  • Individuals currently using a monoamine oxidase inhibitor (MAOI)
  
  
  
  • Individuals with narrow-angle glaucoma
  
  
  
  • Individuals who are breastfeeding
    
• Use: Antihistamines come in tablet, chewable tablet, capsule, and liquid forms. How often an antihistamine should be taken each day depends on the individual antihistamine's characteristics and the type of preparation (that is, the dosage form).



• Drug or food interactions: Avoid taking other drugs that cause drowsiness, such as alcohol, sleep preparations, sedatives, or tranquilizers. Avoid taking MAOIs (for example, isocarboxazid [Marplan], phenelzine sulfate [Nardil], or tranylcypromine [Parnate]) within 14 days of antihistamines. For drug interactions specific to a particular antihistamine, talk with a doctor or pharmacist.



• Side effects: Many antihistamines (particularly the first-generation agents) may cause the following side effects:
  
  
  • Drowsiness
  
  
  
  • Dry mouth
  
  
  
  • Urine retention
  
  
  
  • Blurred vision
  
  
  
  • Before driving a car or operating machinery, be sure to know if the antihistamine affects the ability to concentrate and stay awake.
  
  
  
  • Check with a doctor before taking antihistamines.