Conversion factors (oral opioids)

Oral MME factors: CDC oral MME conversion factors (analytic file). IV/parenteral factors are equianalgesic conventions (McPherson, Demystifying Opioid Conversion Calculations, ASHP) — route ratios vary in the literature; treat as approximations. MME is for risk stratification, not opioid rotation.

How the conversion works — the common currency

Every opioid is first converted into morphine milligram equivalents (MME) — the amount of oral morphine that would give about the same pain relief. Conversions happen in two steps: the current opioid becomes MME, then MME becomes the new opioid. That is why a single table of numbers can handle any drug-to-drug switch. (Why the final dose is then reduced is incomplete cross-tolerance — see §3, Opioid Rotation.)

Why oral doses are larger than injected ones

Swallowed opioids pass through the liver before reaching the bloodstream, and much of the dose is broken down there (first-pass metabolism). Only a fraction reaches circulation — so oral doses are bigger for the same effect. Morphine is roughly 1:3 intravenous (IV) to oral; HYDROmorphone roughly 1:5.

Why the numbers are approximate

Conversion factors are population averages drawn from studies with real variability, and published references disagree — HYDROmorphone IV-to-oral is cited anywhere from 1:2 to 1:5. This tool uses the CDC analytic file for oral factors and standard equianalgesic convention (McPherson/ASHP) for injectable ones, and names the source for each.

Fentanyl IV — when to use a lower factor

Fentanyl's behavior changes with infusion duration. In the first hours, its effect is limited by redistribution into tissue rather than by elimination — the patient has received a large total dose without the sustained exposure that builds tolerance. With continued infusion, elimination becomes clearance-limited and the drug accumulates. The shift is gradual, largely complete by roughly 12–24 hours.

Consider a more conservative factor — as low as 0.1 MME/mcg — when tolerance may not be established, or when the consequences of over-estimating are higher:

• Infusion under ~24 hours, especially if the patient was opioid-naive beforehand
• Age 65 or older
• Hepatic impairment (fentanyl is cleared hepatically via CYP3A4)
• Concurrent benzodiazepines or other central nervous system depressants
• Untreated obstructive sleep apnea or obesity hypoventilation
• Concurrent CYP3A4 inhibitors (clarithromycin, azole antifungals, ritonavir) — a residual-drug problem rather than a tolerance one: inhibited clearance means fentanyl persists after the infusion stops, so a new opioid stacks on drug still on board
• Converting to morphine in renal impairment — active metabolites accumulate

Worked example — a 25 mcg/hr drip (600 mcg/day):

Note the auto reduction tier flips between the two rows — at 60 MME the regimen falls in the ≤75 tier, so the smaller 40% reduction applies. The difference is large, so the choice matters. When in doubt, start at the lower estimate, provide adequate breakthrough medication, and titrate up — under-dosing is recoverable; over-dosing is not. Verify against your institutional protocol.

Why doses round down

When a calculated dose falls between available tablet strengths, this tool rounds down. Under-dosing is recoverable — breakthrough medication covers the gap and the dose is titrated up at follow-up. Over-dosing has no equivalent rescue. (Breakthrough sizing — typically 10–20% of the total daily dose, sized to the regimen the patient will actually receive, not the raw calculated figure — is covered in §3.)

How to calculate total daily MME

• List all scheduled + PRN opioids actually taken
• Each: dose × doses/day × MME factor
• Sum — for risk stratification only, not a rotation conversion
• Example: oxyCODONE 10 mg q8h = 30 mg/day × 1.5 = 45 MME/day

Fentanyl transdermal — special rules

• Change patch q72h (some need q48h); onset 12–24h after first patch — fentanyl must first build a depot in the skin before levels become therapeutic
• After removal the drug keeps being absorbed from that skin depot — serum levels take ~17h to fall by half (often longer), so removal does not stop absorption; cover with IR
• Fever & external heat ↑ absorption — caution with heating pads, hot tubs
• Rescue dosing is individualized: ensure an appropriate immediate-release opioid plan is available during conversion and titration. Do not calculate a prescription by multiplying the patch rate by the CDC 2.4 MME factor or by reversing the FDA patch table; select the drug, dose, and interval from prior response, organ function, sedation risk, and institutional protocol.

Fentanyl rescue and titration principles: current FDA fentanyl transdermal labeling calls for appropriate immediate-release rescue but gives no patch-rate-to-rescue formula; the CDC 2022 guideline says MME factors must not determine doses when converting between opioids. US institutional algorithms commonly use 10–20% only after a specific scheduled opioid regimen is selected. Reviewed August 2026.

Methadone — non-linear

Half-life 24–120h; QTc prolongation (baseline ECG); delayed respiratory depression; titrate no more often than q5–7 days.

Oral ↔ IV/SC ratios

Pain vs. OUD products — don't mix them up

Belbuca (buccal) starting dose — FDA label

• Titrate in increments of 150 mcg q12h, no more often than every 4 days; max 900 mcg q12h (QTc ceiling).
• Buccal bioavailability ~45–50% (vs patch ~15%); halve the dose in severe hepatic impairment or oral mucositis.

Patch (Butrans) & transitions

• Butrans is a 7-day patch in mcg/hr; used for around-the-clock pain in lower-dose/opioid-tolerant pain patients.
• Patch → buccal: remove patch, then begin buccal after ~12–24 h, monitoring for early withdrawal before the first dose; titrate to effect.
• Depot effect: buprenorphine keeps absorbing from skin for hours after patch removal.

Acute pain in a buprenorphine patient

Why no MME factor

As a partial agonist with a ceiling and non-proportional potency, buprenorphine has no validated CDC MME conversion factor. It is therefore handled here as reference/education only — excluded from the MME calculator and not offered as a rotation/IV→PO target, the same principled approach used for methadone.

Belbuca dosing, ≤30 mg MSE taper threshold, titration interval, 900 mcg q12h ceiling, and hepatic/mucositis halving per the FDA Belbuca (buprenorphine buccal film) Prescribing Information, §2.2–2.3 (Endo/BioDelivery Sciences); Butrans (transdermal) PI; CDC 2022 CPG. Buprenorphine has no CDC MME factor. Always confirm the current FDA label before prescribing.