Hypoventilation

• Hypoventilation, abnormally slow or shallow respiration, can occur when taking Fentanyl, and the risk of developing this side effect increases with individuals who do not have a tolerance to opiates.
  
  

Bradycardia

• Bradycardia, an abnormally slow resting heart rate, is an uncommon but potentially serious side effect and is most common with infrequent use of Fentanyl.
  
  

Nervous System

• Impaired motor ability, syncope, tremor, impaired concentration and fatigue may occur with use of Fentanyl. Operating heavy machinery or a motor vehicle should be avoided until the effects are known.
  
  

Urinary Retention

• Difficulty with urinating, leading to urinary retention, is a side effect that occurred in 3 percent to 10 percent of patients taking Fentanyl. If this side effect persists, consult your physician.
  
  

Digestive

• Nausea, upset stomach and constipation are common side effects in users of Fentanyl. These side effects can be bothersome but are non-fatal.
  
  

Addiction

• Fentanyl has a high potential for tolerance, withdrawal and dependence. Before you stop using Fentanyl, consult with your physician regarding an appropriate strategy for cessation.