Antihistamines are a class of medications used to counteract the effects of histamine, a chemical released by the body during an allergic reaction. These drugs also frequently are used to relieve or prevent symptoms associated with colds and the flu.

Some experts have disputed the effect of antihistamines in effectively treating cold symptoms, maintaining that antihistamines work by blocking the effects of histamines, which are not involved in symptoms of colds or the flu. Others argue that antihistamines are an effective way to relieve certain symptoms associated with these respiratory infections. Many cold and flu medications contain antihistamines.

Some of the antihistamine formulations that are often used to treat colds and the flu include:

• Antihistamine/decongestant combinations
  
  
• Antihistamine/decongestant/analgesic (pain relief) combinations
  
  
• Antihistamine/decongestant/anticholinergic (antispasmodic) combinations

Patients should not take antihistamines, unless prescribed by a physician, if they have been diagnosed with certain medical conditions. Certain medications also may negatively interact with antihistamines.

Antihistamines may also produce slightly different side effects depending on the type of antihistamine used. Symptoms of overdose can be similar to the medication’s side effects, but are usually more severe. The side effects and effectiveness of antihistamines may also vary between different age groups.