Most Common Effects

• The most common side effects associated with Benadryl are related to its sedating action. The person might feel tired, drowsy or sleepy, be confused and have problems with coordination.
  
  

Other Side Effects

• Other side effects include blurred or double vision, difficulty urinating, dizziness, heart palpitations, increased heart rate, irritability, low blood pressure, nervousness, tremors, stomach upset, loss of appetite and nausea.
  
  

Uncommon Side Effects

• Some side effects have been reported infrequently. These include chills, constipation or diarrhea, dry mouth, euphoria, fatigue, increased perspiration, sensitivity to sunlight and skin tingling or numbness.
  
  

Drug Interactions

• Benadryl exaggerates the sedating effects of alcohol, benzodiazepine anti-anxiety drugs, narcotic pain medications, tricyclic antidepressants and some blood pressure medications.
  
  

Contraindications

• People with certain conditions may not be able to use Benadryl. These include asthma, cardiovascular disease, enlarged prostate gland, glaucoma, high blood pressure and hyperthyroidism.