Opioid Conversion
Calculator

Opioid equianalgesic conversion is the process of translating a current opioid regimen into an equivalent dose of a different opioid, maintaining analgesic effect while rotating away from a drug causing intolerable side effects, accessibility issues, or end-of-life management changes.

The common currency of opioid conversion is the oral morphine equivalent (OME) — also called the morphine milligram equivalent (MME) — which expresses any opioid dose in terms of the equivalent amount of oral morphine. This standardised unit allows doses of different opioids across different routes to be compared and safely converted.

Accurate opioid conversion is essential in cancer pain management, palliative care, chronic non-cancer pain transitions, acute pain management (e.g., transitioning from IV to oral), and addiction medicine (where total opioid load informs treatment decisions).

Add one or more source opioids — selecting the drug, dose per administration, and frequency per day. The calculator converts each to oral morphine equivalents (OME) using the drug-specific conversion coefficient and sums them to generate the total 24-hour OME.

Select a target opioid and the calculator applies the appropriate conversion coefficient in reverse, generating the 24-hour target dose. A dose reduction of 25–50% is applied by default for cross-tolerance (incomplete tolerance between opioids), which is a standard safety measure in opioid rotation.

Conversion coefficients are editable — the calculator loads illustrative placeholder values by default, but requires the clinician to verify and replace these with values from their institution's authoritative source before use.

Persistent high-risk warnings appear for: methadone (non-linear pharmacokinetics make conversion ratios highly variable and weight- and dose-dependent — always requires specialist input); transdermal fentanyl (µg/hour to OME conversion varies significantly by brand and clinical guidelines).

Multi-drug entry supports complex pain regimens where patients are receiving more than one opioid simultaneously — all entries are converted to OME and summed before the target dose is calculated.

The conversion process follows three steps: source to OME, dose reduction for cross-tolerance, and OME to target.

The following coefficients represent commonly cited illustrative values and must be verified against institutional formulary. Different published references and national guidelines may cite different values for the same drug, particularly for parenteral routes.

Oral morphine: coefficient 1 (reference standard). Oral oxycodone: approximately 1.5 (i.e., 10 mg oxycodone ≈ 15 mg oral morphine). Oral hydromorphone: approximately 4. Oral codeine: approximately 0.15. IV/SC morphine: approximately 3 (parenteral routes are more bioavailable).

Methadone is uniquely complex — its coefficient is not fixed but varies between 4 and 20 depending on the total daily OME, with higher OME doses requiring a higher conversion ratio. Use of a table-based conversion (e.g., from the BNF or ANZCA guidelines) is mandatory; a fixed coefficient approach is unsafe for methadone.

Transdermal fentanyl is expressed in µg/hour rather than mg — a patch delivering 25 µg/hour corresponds to approximately 60–90 mg oral morphine per 24 hours depending on the reference used. Always use the guideline-specific conversion table.

Always apply a cross-tolerance reduction (25–50%) when rotating between opioids. Patients are not fully cross-tolerant between opioids — incomplete cross-tolerance means the new drug at the calculated equianalgesic dose may have greater effect than expected.

Titrate to effect after rotation. The initial post-rotation dose is a starting point. Provide breakthrough doses (typically 1/6 of the 24-hour dose) and closely monitor for both under-analgesia and toxicity in the first 24–48 hours.

Seek specialist input for high-risk rotations: switching from any opioid to methadone, switching at very high OME doses (>300 mg/day), patients with renal or hepatic impairment, and frail or elderly patients.

Document clearly. Record the source opioid(s), conversion rationale, coefficients used and their source, the calculated dose before and after reduction, and the final prescribed regimen. Clear documentation prevents errors during subsequent clinical handovers.