Patients using antihistamines should consult their physicians before taking any additional prescriptions, over-the-counter medications, nutritional supplements or herbal remedies. Of particular concern to individuals taking antihistamines are:

• Erythromycin. A type of antibiotic used to fight many types of bacterial infection. Use of this drug with an antihistamine can lead to an elevated amount of certain types of antihistamines in the blood, and may increase the severity of side effects.
  
  
• Anticholinergics (antispasmodics). Medicine used to treat airway, abdominal and stomach spasms as well as cramps. Anticholinergics can cause drowsiness (a common side effect also associated with antihistamines) and therefore should not be combined with antihistamines. However, it should be noted that some medications are formulated to contain both antihistamines and anticholinergics. Patients are urged to consult a physician before using these or any other medications.
  
  
• Monoamine oxidase (MAO) inhibitors. Medications used in the treatment of depression. They can cause severe drowsiness when taken with antihistamines.
  
  
• Central nervous system depressants. Used to treat anxiety, muscle tension, pain, insomnia, acute stress reactions, panic attacks and seizure disorders by slowing down brain activity. Using these drugs with antihistamines may worsen the side effects associated with both drugs.
  
  
• Antifungals. Medicine used to treat fungal infections. Certain antifungals should not be used in combination with antihistamines. For instance, ketoconazole can lead to an elevated amount of the antihistamine fexofenadine in the blood. Itraconazole should also be avoided.
  
  
• Aspirin. Symptoms associated with the effects of using large amounts of aspirin can be masked when using antihistamines.
  
  
• Alcohol. Effects associated with the consumption of alcohol can be increased with the use of antihistamines.