Antihistamines can aggravate certain medical conditions in some patients. Although physicians may prescribe antihistamines to patients with some of these conditions, patients should avoid over-the-counter antihistamines if they have been diagnosed with any of the following conditions:

• Asthma. Antihistamines can provoke an asthma attack in some individuals.
  
  
• Ear infection. Antihistamines can make an ear infection worse and/or lengthen the time it takes to resolve.
  
  
• High blood pressure (hypertension). Decongestant and antihistamine combinations can elevate blood pressure. Individuals with hypertension should avoid both types of medications.
  
  
• Enlarged prostate, intestinal obstruction or urinary tract block. Antihistamine use can aggravate these conditions.
  
  
• Heart problems. Antihistamines can produce heart palpitations (strong, fast, irregular or abnormal heartbeats) or arrhythmias (abnormal heartbeats that may be unusually fast or unusually slow) in individuals with heart conditions.
  
  
• Thyroid conditions. Thyroid conditions can lead to an elevated heartbeat, which can be further complicated with antihistamine use.
  
  
• Liver, heart, lung or kidney disease. Antihistamines may cause increased enzyme production that can interfere with these conditions.
  
  
• Glaucoma. Antihistamine use can aggravate glaucoma, a disease that causes increased pressure in the eye.