Fentanyl

Class: analgesic – opioid – full opioid agonist

Indications: step 3 on the WHO ladder for severe pain, anaesthetic premed

Contraindications/cautions: fentanyl hypersensitivity/allergy (not nausea/hallucinations)

Adverse reactions: see morphine – less constipating (reduce dose of laxatives when converting from morphine), perhaps less sedating and less emetogenic than other opioids

Metabolism/clearance: metabolised by metabolising enzyme CYP3A mainly in the liver

Interactions:

• increased clinical effect/toxicity of fentanyl (due to increased blood concentrations) may occur with some CYP metabolising enzyme inhibitors (see above) e.g. aprepitant, clarithromycin, grapefruit juice, indinavir, itraconazole, ketoconazole, nelfinavir, ritonavir, telapravir, voriconazole
• decreased clinical effect/toxicity of fentanyl (due to decreased blood concentrations) may occur with some CYP metabolism enzyme inducers (see above) e.g. carbamazepine, phenobarbitone, phenytoin, rifampicin, St John’s wort
• additive CNS depression with other CNS depressants e.g. benzodiazepines (e.g. lorazepam), phenothiazines (e.g. chlorpromazine), tricyclic antidepressants (e.g. amitriptyline), other opioids, alcohol

Syringe driver: see syringe driver compatibility chart

Mechanism of action: stimulates opioid receptors in the CNS and gastrointestinal tract

Peak effect: patch: 12 to 24 hours after initial application

Duration: patch: 72 hours (plus depot effect see later)

Notes:

• patches are unsuitable for opioid naïve patients. Initial dose based on previous 24 hour opioid requirement
• allow patch to be applied for 24 hours before evaluating analgesic effect; phase out previous analgesic gradually
• adjust dose at 48 to 72 hour intervals in steps of 12.5 micrograms subcut /hour to 25 micrograms subcut /hour
• if patient is hot, or there is a heat pad near the patch, rate of absorption may increase
• if patch comes unstuck use MicroporeTM round edges to reattach
• for acute toxicity give naloxone 2 mg and repeat as required (max 10 mg) over a prolonged time (depot in skin – see below)
• patches leave a depot in the skin which will carry on releasing fentanyl after removal (at least 17 hours for concentrations to drop by 50%)
• use another opioid or the fentanyl injection subcut/sublingual/intranasal for breakthrough – for fentanyl the dose may not relate to background so start at 25 micrograms fentanyl and titrate to effect
• approximate conversion is morphine (po): fentanyl (subcut/patch) = 150:1 i.e. 10 mg morphine po = 66 micrograms fentanyl subcut but in chronic use this can only be used as an estimate

Conversion Chart