Switching a patient from one opioid to another — an opioid rotation — is one of the higher-risk routine tasks in pain and palliative care. Getting the dose wrong in either direction causes uncontrolled pain or, worse, overdose. This calculator applies a standard equianalgesic table to estimate an equivalent dose and then applies a mandatory cross-tolerance reduction, so it never presents a bare equivalence as a safe starting dose.
How the conversion works
Every opioid in the table carries an oral-morphine-equivalent (OME) factor: the amount of oral morphine equivalent to 1 mg of that drug. A rotation is a two-step conversion — the source dose is multiplied by its factor to get the total oral morphine equivalent, then that equivalent is divided by the target drug's factor to get the raw equianalgesic dose.
The factors come from a standard palliative-care equianalgesic table (Fine PG, Portenoy RK; Palliative Care Network of Wisconsin; StatPearls "Opioid Equivalency"). These tables are derived from limited single-dose and steady-state studies and vary between sources, so the output is an approximation to start from, not a precise prescription.
Why the cross-tolerance reduction is mandatory
Tolerance to one opioid does not transfer completely to another. If you start the new opioid at its full equianalgesic dose, the patient may be effectively over-dosed because they are less tolerant to the new drug than the math assumes. Guidelines therefore recommend reducing the calculated dose by 25–50% — more for high starting doses, frail or elderly patients, or when pain is already well controlled — and then titrating upward as needed with breakthrough coverage. This calculator defaults to the conservative 50% reduction and shows the 25% and 50% options side by side.
High-risk agents and hard limits
Two agents are handled specially. Methadone has a non-linear, dose-dependent conversion, a long and variable half-life, and QT-prolongation risk; a simple ratio can be dangerously wrong, so this tool does not output a methadone dose at all and instead directs you to a specialist. Transdermal fentanyl is dosed off the 24-hour oral-morphine equivalent (roughly 2.4 mg oral morphine per 1 mcg/hr), must be rounded to an available patch strength, and is never appropriate for opioid-naive patients. Tramadol is a weak, atypical opioid that equianalgesic ratios capture poorly. In every case the calculator is an educational aid: the actual rotation must be performed or verified by a licensed prescriber, with each dose checked against current prescribing information.