Function

• Antihistamines generally are over-the-counter medications used to prevent allergy and flu symptoms. They can also be used to prevent motion sickness and the allergic reactions to insect bites and plants like poison ivy and poison oak.
  
  

Side Effects

• Some antihistamines can make you feel drowsy, an effect that is enhanced by drinking alcohol. Antihistamines may also cause dry mouth and dry eyes.
  
  

Types

• Antihistamines are available alone, but are often combined with cough, cold and flu medicines. If you have flu, cold or allergy symptoms avoid doubling up on medication that may contain antihistamines. Excessive use of antihistamines can increase the intensity of unpleasant side effects, such as those mentioned above, and can lead to sinus infections. If in doubt about whether a medication contains antihistamines, ask a physician or pharmacist.
  
  

Considerations

• Antihistamines are generally well tolerated. However, those who take sleeping pills or sedatives, muscle relaxant drugs, or medicine to treat blood pressure should speak with a doctor or pharmacist before taking an antihistamine.
  
  

Warning

• In addition to drug interactions, different antihistamines can cause different side effects. Prior to taking an antihistamine for the first time, it is important to read the "Warnings" or "Precautions" on the packaging.