PharmacologyTier 1Medical Sciences concept

Opioid conversion - oral morphine equivalent dosing

Core concept

All opioids converted to a common reference (oral morphine equivalent daily dose, OMEDD) to compare potency and assess overdose/dependence risk
Conversion is via published equianalgesic ratios, not linear across the whole dose range -- ratios are population averages with wide individual variability
Incomplete cross-tolerance between opioids means a calculated equivalent dose should be reduced by 25-50% when switching agents, even in a tolerant patient

Approximate oral potency relative to oral morphine (illustrative ratios only -- always use a current local reference, not a memorised number, for real dosing): oxycodone roughly 1.5x morphine; oral morphine to transdermal fentanyl and methadone conversions are markedly non-linear and dose-dependent -- methadone in particular becomes disproportionately more potent at higher cumulative morphine-equivalent doses
Route matters independently of drug: oral vs parenteral morphine differ by roughly 2-3x due to first-pass metabolism -- always convert route before drug
OMEDD thresholds (e.g. commonly cited monitoring/caution levels) are used for opioid stewardship and overdose-risk flagging, not for calculating an individual's actual dose requirement

Never use a linear equianalgesic table for methadone -- the morphine:methadone ratio rises steeply with increasing pre-conversion morphine dose; methadone conversion requires specialist input
Always apply a safety reduction (25-50%) for incomplete cross-tolerance when switching opioids or routes, then titrate up against the patient's response -- do not dose the full calculated equivalent
High OMEDD (e.g. >100 mg/day oral morphine equivalent, a commonly cited caution threshold) warrants naloxone co-prescription discussion and review of ongoing need

Same conversion logic underlies recognising iatrogenic overdose risk when multiple opioids/patches are prescribed concurrently without dose reconciliation
Renal impairment separately affects opioid choice via active metabolite accumulation (see morphine-6-glucuronide note) -- a distinct issue from equianalgesic conversion
Buprenorphine's partial agonism/high receptor affinity makes standard equianalgesic tables unreliable for conversion onto or off it -- requires specific protocols

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