Patients should not take antihistamines, unless prescribed by a physician, if they have been diagnosed with any of the following conditions:

• Asthma (chronic inflammation of the airway tissues) and other chronic lung disorders. Antihistamines can worsen chronic lung disorders such as asthma, emphysema (condition in which the air sacs in the lungs are damaged or collapse, interfering with the normal exchange of oxygen and carbon dioxide) and chronic bronchitis (a condition in which the breathing passages to and from the lungs have become irritated, decreasing air flow and resulting in greater mucus production). Antihistamines may dry the mucus in the chest and lead to breathing problems. Antihistamines can provoke an asthma attack in some individuals.  
  
  
• Ear infection (otitis media). Antihistamines can make an ear infection worse and/or lengthen the time it takes to resolve.
  
  
• High blood pressure (hypertension). Medications that combine antihistamines with decongestants can elevate blood pressure. Individuals with hypertension should avoid both to be safe.
  
  
• Enlarged prostate, intestinal obstruction, stomach ulcer or urinary tract block. Antihistamine use can aggravate these conditions.
  
  
• Heart problems. Antihistamines can produce heart palpitations (an awareness of a strong, fast, irregular, abnormal or "galloping" heartbeat) or arrhythmias (an abnormal heartbeat 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 can cause increased enzyme production that can interfere with these conditions.
  
  
• Glaucoma (disease characterized by increased pressure in the eyeball). Antihistamine use can aggravate glaucoma.