What is oral morphine equivalent (OME)?

**Oral morphine equivalent (OME)**, also called morphine milligram equivalent (MME), is a standardised unit that expresses any opioid dose as the equivalent amount of oral morphine. It allows different opioids given via different routes to be compared on a common scale.

Why do I need to reduce the dose when rotating opioids?

Because opioid tolerance is *incomplete cross-opioid tolerance* — the patient's tolerance to the new opioid is lower than to the drug they are rotating from. Giving the full equianalgesic dose risks **over-sedation and respiratory depression**. A 25–50% reduction creates a safety margin, with the dose titrated up if analgesia is inadequate.

Why is methadone conversion so different?

Methadone has *non-linear pharmacokinetics*, a very long and variable half-life (15–60+ hours), and a conversion ratio that changes with total opioid dose. At low OME doses the ratio may be 4:1; at high doses it can exceed 20:1. A fixed coefficient approach for methadone is clinically unsafe — always use specialist guidance and a reference-specific conversion table.

How do I convert transdermal fentanyl to oral morphine?

A fentanyl patch rated at **25 µg/hour** corresponds to approximately *60–90 mg of oral morphine per 24 hours* — the exact ratio varies by guideline (ANZCA, BNF, and others differ). Never estimate this from first principles alone. Use the conversion table from your institutional formulary and seek pain or palliative care specialist input.

Can I use this calculator for breakthrough dose calculation?

**Breakthrough doses** are typically *1/6 of the total 24-hour opioid dose* (oral morphine equivalent), available every 1–4 hours as needed. Once the 24-hour OME is calculated, the breakthrough dose is straightforward to derive — but the specific drug, formulation, and interval must be prescribed by the clinical team.

What is the difference between opioid rotation and opioid switching?

The terms are often used interchangeably. **Opioid rotation** typically refers to a *planned transition* to a different opioid (often for side effect management), while **opioid switching** can refer to any change — including route changes (e.g., IV to oral). Both processes use OME conversion as their basis.

Pharmacology · Pain Management

Opioid Conversion
Calculator

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⚠️ High-Risk Tool — Important Notice

Last reviewed: March 2026

Opioid conversions are high-risk clinical decisions. Errors can cause serious harm including overdose or undertreated pain.

The conversion coefficients in this tool are illustrative placeholders only. They have NOT been verified for clinical use. You must replace them with values from your institution's approved formulary or a validated clinical reference before using any result in patient care.

Methadone and transdermal fentanyl conversions require specialist oversight and are not suitable for estimation using simple OME ratios.

Always involve a clinical pharmacist and/or pain/palliative care specialist for opioid rotations in complex patients.

I understand the conversion coefficients are placeholders and must be verified before any clinical use.