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

• Asthma. 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). Decongestant and antihistamine combinations 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 (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 (a group of eye diseases that affect the optic nerve, which connects the eye to the brain). Antihistamine use can aggravate glaucoma.