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 some antihistamines in the blood.
  
  
• Anticholinergics. Medicine used to treat abdominal and stomach spasms as well as cramps. Anticholinergics can cause drowsiness (and other side effects also associated with antihistamines) and therefore should not be combined with antihistamines.
  
  
• 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 this drug with antihistamines can 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 fexofenadine in the blood when taken with the antihistamine. Itraconazole should also be avoided.
  
  
• Aspirin. Symptoms associated with the use of 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.