Conversion calculations between opioids - fentanyl

Formulations

Fentanyl - a potent synthetic opioid, available as a transdermal patch for stable chronic pain and as fast-acting formulations (sublingual, buccal, intranasal) for breakthrough pain - pharmacokinetically distinct from oral opioids given its lipophilicity and non-oral routes

Particularly favoured in renal impairment (unlike morphine, fentanyl has no active renally-cleared metabolites requiring accumulation caution) and for patients unable to swallow oral medication reliably

Highly lipophilic, enabling transdermal absorption; hepatically metabolised via CYP3A4 to inactive metabolites - explains its relative safety in renal impairment compared with morphine, and its relevant interaction risk with CYP3A4 inhibitors (which can precipitate toxicity) or inducers (which can reduce efficacy)

Transdermal patches are unsuitable for acute/unstable pain or initial opioid titration - slow onset (12-24h to reach steady state) and slow offset (residual effect for many hours after patch removal, from the cutaneous depot) mean transdermal fentanyl is reserved for patients with stable, well-characterised opioid requirements, not for opioid-naive patients or rapidly changing pain

Patch strength selection is based on prior opioid requirement (using an established conversion table/tool, e.g. 25microg/hr patch approximating a specified oral morphine equivalent daily dose - exact figures vary by reference source and must be checked against the current local guideline)
Fever/external heat (heat packs, hot baths, high fever) increases transdermal absorption and can precipitate toxicity - specific patient education point
Breakthrough pain still requires a separate, appropriately dosed fast-acting opioid alongside the patch, not patch strength escalation alone

Renal impairment (a specific preferred-opioid scenario), CYP3A4 drug interactions, heat-related absorption changes, cachexia/reduced subcutaneous fat (can alter transdermal absorption reliability)

Because of its slow onset/offset kinetics, fentanyl patches are a common source of dosing error when clinicians attempt to titrate them like an oral opioid - dose changes should be made cautiously and infrequently (typically not more often than every 72 hours, matching the patch's own duration), with breakthrough medication used to bridge in the interim

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