Addiction

• Addiction is a possible side effect of the fentanyl patch, according to MedlinePlus. Those who have a history of abusing alcohol or drugs should not use this treatment.
  
  

Overdose

• Fentanyl patches release small amounts of medication into the body over a three-day period, according to MedlinePlus. If the patch breaks or the patient is sensitive to the drug, fatal overdose can occur.
  
  

Allergic Reaction

• Allergic reaction, which can be fatal, can happen with a fentanyl patch, according to MedlinePlus. Symptoms include breathing problems, redness, itching and rash.
  
  

Psychological Issues

• Possible psychological side effects include hallucinations, delusions, mood changes, depression and nervousness, according to MedlinePlus.
  
  

Physical Symptoms

• Potential physical side effects associated with treatment include stomachache, sleeping problems, weakness, dizziness, diarrhea and constipation, according to MedlinePlus.