Fentanyl is not suitable for use in some patients predominately because of well recognised opioid side effects and aggravation of existing medical conditions. Fentanyl may lead to severe adverse effects in patients with COPD or other respiratory conditions by decreasing respiratory drive and increasing airways resistance. Fentanyl can cause bradycardia and should be used with caution in patients who suffer from arrhythmias associated with a reduced heart rate. Fentanyl is metabolised in the liver and hepatic impairment may slow the elimination of metabolites. Consequently, patients with impaired hepatic function prescribed fentanyl should be monitored closely for signs of fentanyl toxicity and the dose of the patch reduced if necessary.

Effect of heat on transdermal fentanyl

The release of fentanyl from transdermal patches is increased by heat. Patients should be advised not to apply heat to the patch such as a hot water bottle, heat pad, electric blanket, heat lamp or tanning lamp and also to avoid generating internal heat through taking hot baths, showers or sunbathing. Patients suffering from a fever or high temperature should be monitored closely for respiratory depression or other signs of adverse effects or overdose.¹⁰

Serotonin syndrome

Serotonin syndrome is a potentially fatal interaction that can occur when fentanyl is co-prescribed with drugs that increase serotonergic activity. Fentanyl is thought to inhibit the reuptake of serotonin, so when used with drugs such as the SSRIs there is potential for an additive effect.

Levels of serotonin in the brain become raised resulting in the symptoms of serotonin syndrome. Symptoms can occur very quickly, in some cases within six hours of taking the first dose of medication. Serotonin syndrome is characterised by three main groups of symptoms⁹:

• Altered mental status
  • Agitation
  • Confusion
  • Mania
    
    
• Autonomic dysfunction
  • Excessive sweating
  • Diarrhoea
  • Fever
  • Shivering
    
    
• Neuromuscular side effects
  • Muscle spasm
  • Tremor
  • Overactive reflexes
  • Incoordination
  • Muscle twitching

Pharmacists are advised to exercise caution when supplying fentanyl to patients already taking serotonergic drugs.

Serotonin syndrome should be considered in patients asking about unusual reactions to newly initiated medication. Patients suspected of suffering from serotonin syndrome should have all serotonergic medication stopped and be monitored closely. 

⁹ Stockley Drugs that cause serotonin syndrome - accessed via medicinescomplete August 2018.
¹⁰ Fentanyl patches; serious and fatal overdose from dosing errors, accidental exposure and inappropriate use. MHRA Sept 2008 http://webarchive.nationalarchives.gov.uk/20141205150130/http://www.mhra.gov.uk/Safetyinformation/DrugSafetyUpdate/CON087796