Opioid Dose Conversion and Morphine Equivalent Calculator

Clinical Context and Pearls

Opioid conversion is the arithmetic in the middle of a clinical decision, and it is the part most likely to be done from memory at the end of a ward round. The numbers below are the ones this tool uses, with the document each came from, because the published ratios do not all agree and a resident should be able to see where the disagreement lies rather than inherit one side of it.

1. Everything Goes Through the Oral Morphine Equivalent

What it is. The oral morphine equivalent daily dose, written OMEDD here, is the amount of oral morphine that would give the same analgesia as everything the patient is actually taking in 24 hours. Every conversion in this tool is done in two steps: each current opioid is converted into oral morphine, the results are added, and the total is then converted out to the target drug and route.

Why it matters. A patient on a fentanyl patch, a sustained-release morphine tablet and eight tramadol capsules a day is on three drugs and one opioid load. Convert each pair directly and you need a table of every combination, and you get a different answer depending on the order you work in. One common currency removes both problems. It is why the guidelines, the labels and the regulators all use it.

Bedside pearl. Add the rescue doses actually taken into the 24 hour total before you convert. The commonest conversion error on a ward is not a wrong ratio; it is converting the prescribed background dose and leaving out six rescues, which under-doses the patient by a third on the day they change over.

A caution about the same number used for a different purpose. The morphine equivalent is also used as a regulatory and overdose-risk metric in chronic non-cancer pain. The CDC states plainly that its conversion factors are not to be used to work out the dose when switching one opioid for another. This tool is built for cancer and palliative pain and does the second job, not the first.

2. The Ratios This Tool Uses, and Where the Sources Disagree

Multiply the 24 hour dose by the factor to get the oral morphine equivalent. Divide by it to go the other way.

Bedside pearl. The direction you convert in changes which disagreement hurts you. Coming off hydromorphone, the difference between a factor of 4 and a factor of 7.5 is nearly double the morphine dose. Going on to it, the same spread halves it. Where two conventions exist, this tool prints both figures and uses the one an Indian guideline states.

3. Incomplete Cross-Tolerance, and the One Case Where You Do Not Reduce

What it is. A patient tolerant to one opioid is only partly tolerant to the next. The receptor occupancy, the metabolite profile and the affinity all differ, so the calculated equianalgesic dose of the new drug behaves as though it were larger. The published answer is to take 25 to 50 per cent off the calculated dose and titrate back up.

Where the numbers come from. The ISSP 2020 guideline states 25 to 50 per cent. WHO 2018 reserves a reduction of around 50 per cent for defined circumstances: a total dose of 1 g of oral morphine equivalent or more in 24 hours, an elderly or frail patient, a switch made because of intolerable effects such as delirium, or a recent rapid escalation that may itself represent opioid-induced hyperalgesia. The Faculty of Pain Medicine sets its high-dose threshold lower, at 500 mg in 24 hours. This tool uses the lower of the two.

The case where you do not reduce. A change of route in the same molecule is not an opioid switch. Oral morphine to subcutaneous morphine is a bioavailability correction, and the factor of 2 already contains it; taking a further 25 per cent off leaves a dying patient in pain on the day the syringe pump goes up. The same holds for immediate release to sustained release. This tool applies no reduction when the molecule does not change, and says so in the result.

The counter-evidence, stated because it cuts against the rule. A 2018 systematic review of opioid rotation in cancer pain found that in every study it examined except those involving methadone, the calculated dose of the second opioid had to be increased to achieve adequate analgesia rather than decreased. Where the reason for switching is that the pain is not controlled, reducing hard is the wrong instinct. This tool reduces least in exactly that case.

4. The Breakthrough Dose, and What Six Rescues in a Day Is Telling You

The rule. The rescue dose is one sixth of the total daily dose, which is the same as one four-hourly dose. The ISSP guideline states it in that form and adds a modification found in no other guidance read for this tool: in renal impairment, a rescue as small as one twelfth of the 24 hour dose may be used. Scottish palliative guidance gives a range of one sixth to one tenth.

How often. The sources disagree, and the disagreement is substantive. The ISSP guideline allows a rescue as often as hourly. Northern Ireland guidance says two to four hourly. A pragmatic reading is that the interval should match the time to peak effect by that route: about an hour for oral immediate release, about twenty minutes for subcutaneous.

Bedside pearl. More than six rescues in 24 hours is not a rescue problem, it is a background dose problem. Scottish guidance asks for review after six doses in a day, and urgent review if three doses in four hours have not helped, because at that point the pain is either not opioid-responsive or something has changed. Count the rescues before you increase the background dose, then add them into the new total: that is the titration.

5. Transdermal Fentanyl: The Lag, The Depot, and Reading the Table Backwards

The lag. Serum fentanyl rises gradually after the first patch and levels off between 12 and 24 hours. The ISSP guideline therefore says to continue the regular four-hourly oral opioid for 12 hours after the patch goes on. Full equilibrium on a new patch strength takes up to six days, so a patch is not the drug to reach for in a pain crisis.

The depot. A reservoir of fentanyl remains in the skin after the patch is taken off. Serum concentrations fall by about half in 17 hours by one section of the label and in 20 to 27 hours by another, and significant absorption continues for 24 hours or more. A patient who becomes toxic does not become safe when the patch is removed.

Reading the table backwards. Both the US label and the UK SmPC state that their conversion tables must be used only to convert on to the patch, never off it, because the tables are deliberately conservative and reversing them overestimates the new drug. That instruction is written for a chronic pain population. In palliative practice a patch sometimes has to come off, and this tool will do the arithmetic, but it prints the label position, applies the larger reduction, and reminds the reader of the depot. Use it as an estimate to be titrated against, not as a prescription.

Indian context. Patch strengths listed by an Indian manufacturer are 12.5, 25, 50, 75 and 100 mcg/h; National Cancer Grid guidance names the lowest as 12 mcg/h. Patches are changed every 72 hours. Heat raises absorption, so a fever, a hot water bottle or a heating pad over the patch is a real dose increase.

6. Buprenorphine: A Partial Agonist, Two Published Ratios and a Ceiling

What it is. Buprenorphine is a partial agonist at the mu receptor with very high affinity and slow dissociation. That buys a favourable safety profile in respiratory terms, and a long duration. It is also why the CDC leaves buprenorphine out of its morphine equivalent table altogether: a partial agonist with a ceiling effect will not sit on the same linear scale as a full agonist.

The two ratios. For the transdermal preparation the ISSP guideline uses 75:1, while WHO and the Faculty of Pain Medicine work at 100:1. Neither the US label nor any UK patch SmPC read for this tool states a ratio at all. The sublingual figure of 75 is a different number for a different preparation. The two are easily conflated.

The ceiling that matters in India. Transdermal strengths confirmed on an Indian manufacturer's list are 5, 10 and 20 mcg/h. At the ISSP ratio, 20 mcg/h is about 36 mg of oral morphine a day. The US label caps treatment at one 20 mcg/h patch because of QT prolongation and says that above 80 mg of oral morphine equivalent an alternative analgesic should be considered. The higher strengths used elsewhere, 35, 52.5 and 70 mcg/h, were not confirmed as marketed in India when this tool was written. Buprenorphine is therefore a step 2 to early step 3 option here, not a way to manage a large opioid requirement.

Bedside pearl. The ISSP guideline places buprenorphine as a fourth line drug for cancer pain. It earns its place mainly in renal impairment, where it has no active metabolite that accumulates.

7. Methadone: Why There Is No Single Ratio

The pharmacology. Methadone has an elimination half-life reported between 5 and 130 hours, averaging about 22, while its analgesic effect after repeated dosing lasts 8 to 12 hours. Steady state takes 5 to 14 days. The consequence is the central risk with this drug: the dose that looks correct on day two can be fatal on day seven. It is also an NMDA receptor antagonist, which is why it can reverse tolerance acquired to a full agonist, and why the conversion ratio rises with the dose the patient was already on instead of staying fixed.

The published bands. The ISSP 2020 guideline gives three, and this tool uses them.

Other published sets run from 3:1 to 20:1 across six bands. A widely reproduced Australian table reaches 12:1 only at 601 to 800 mg and goes on to 20:1 above 1000 mg. A separate family of tables from chronic non-cancer pain practice reaches 12:1 at 60 mg a day, an order of magnitude lower, and must never be used for a cancer pain patient. Above roughly 1000 to 1200 mg of oral morphine equivalent the ratio tables have no useful empirical support, and the published alternative is a fixed ceiling of about 30 mg of methadone a day rather than a ratio at all.

The QT interval. The S-enantiomer binds the hERG channel and prolongs the QT. The thresholds quoted in the Indian methadone literature, from the 2014 American Pain Society safety guideline, are a QTc above 450 ms as a signal to reconsider and 500 ms as a reason to withhold or discontinue, along with a rise of more than 60 ms from the patient's own baseline. An ECG within the last three months with a QTc under 450 ms can serve as the baseline. At the end of life, where the goal is comfort, the same guidance permits an informed conversation with the patient and family and continuing methadone without subjecting them to further investigation.

Bedside pearl. Every source read for this tool says the same thing in different words: methadone is started by somebody experienced with it, and one guideline declines to publish any ratio at all on the grounds that a number invites a calculation where a clinical judgement is required. Have a colleague repeat the arithmetic independently before the first dose is given.

8. The Kidney and the Liver

Why morphine is the problem drug in renal failure. Morphine is glucuronidated to morphine-6-glucuronide, which is an analgesic many times more potent than morphine itself, and morphine-3-glucuronide, which is not analgesic and is neurotoxic. About 90 per cent of the conjugates leave by the kidney. In renal failure the half-life of the glucuronides has been reported to rise from about 4 hours to between 14 and 119. The patient becomes drowsy, myoclonic and confused on a dose that was correct last week.

What to use instead. Fentanyl, buprenorphine and methadone are the lipophilic opioids without renally cleared active metabolites, and are the ones recommended when the eGFR falls. The evidence position should be stated honestly: a 2017 systematic review concluded that the preference rests on pharmacokinetics and clinical experience rather than on trials, and that no recommendation on a preferred opioid could be formulated from the evidence.

Codeine and hydromorphone are avoided in stage 4 and 5 chronic kidney disease in Scottish guidance, although a 2024 American source takes the opposite view on hydromorphone and considers it preferable to morphine. Where a syringe pump is needed at a very low eGFR, UK guidance names alfentanil. This tool does not convert to alfentanil: it is not among the six notified Essential Narcotic Drugs and no Indian guideline read for this tool names it. The threshold at which UK guidance switches to alfentanil has itself moved, from an eGFR below 30 in the 2019 document to 20 or less in the current one.

The liver. Codeine, tramadol and tapentadol are prodrugs, or depend on hepatic activation, so in liver failure they accumulate and stop being reliable analgesics at the same time. Pethidine should be avoided outright: seizures from normeperidine occur even at reduced doses. In severe impairment, oxycodone is started at 30 to 50 per cent of the usual dose and hydromorphone at 50 per cent, while morphine is used at lower doses but normal intervals in early liver disease and at longer intervals as it advances. Fentanyl is described by some as the preferred opioid in liver failure, on empirical grounds rather than trial evidence.

9. Constipation, Nausea and Titrated Naloxone

The standing laxative. Constipation affects nearly every patient on a strong opioid, does not improve with time, and is the commonest reason a patient stops taking a drug that was working. NICE directs that laxative treatment be prescribed for all patients, regularly and at an effective dose. Indian and UK formulary guidance specify a softener together with a stimulant. It is prescribed with the opioid, not after the patient reports the problem.

Nausea. Usually settles within a week of starting or of a dose increase, and an antiemetic can be given for that period rather than indefinitely. Nausea that persists beyond a week deserves another explanation.

Naloxone, given the way palliative practice gives it. A patient on regular opioids for pain is physically dependent, and a full 400 mcg bolus produces acute withdrawal and the sudden return of severe pain, which is its own emergency. The palliative method is to dilute one 400 mcg ampoule to 10 mL with normal saline and give small increments slowly, every 2 minutes, titrating to the respiratory rate and not to the level of consciousness. The increment differs between guidelines: 20 mcg, 80 mcg and 100 mcg are all published from the same dilution. Most patients respond after 80 to 160 mcg in total. Reserve the full undiluted bolus for a respiratory rate that is very low or absent.

Bedside pearl. Naloxone has a shorter duration than most of the opioids it reverses, and far shorter than a fentanyl patch depot or methadone. One good response is not the end of the episode: the patient needs continued observation and often a repeat dose or an infusion.

10. Prescribing Opioids in India: The NDPS Amendment of 2014

What changed. The Narcotic Drugs and Psychotropic Substances Act 1985 was amended in 2014 to create a category of Essential Narcotic Drug, defined in section 2 as a narcotic drug notified by the Central Government for medical and scientific use. The effect was to move these drugs from a patchwork of state licences to one central rule set, which is the single most important change to opioid access in Indian palliative care.

The drugs. The notified list carries morphine, codeine and ethyl morphine, fentanyl, oxycodone, methadone and hydrocodone. Note a discrepancy in the official literature rather than papering over it: the Ministry of Health and Family Welfare operational guidelines of 2017 list five and omit methadone, which was approved for sale for pain management in India in 2016. Of the drugs on the list, oxycodone and hydromorphone were reported as not marketed in India by the ISSP guideline in 2020.

The institution, not the doctor, holds the licence. A government, municipal or Zilla Parishad hospital or dispensary with at least one registered medical practitioner trained in pain relief and palliative care is deemed a Recognised Medical Institution under Rule 52N, without applying for it. A private institution applies to the State Drug Controller on Form 3F, is inspected, and expects a decision within 60 days; recognition runs in renewable three year periods and the certificate is Form 3G. The practical prerequisites are a trained doctor in charge, double locked storage, and the ability to keep a consumption register.

What the prescription must carry. Rule 52G requires that it be in writing, dated and signed, with the prescriber's full name, address and registration number, the name and address of the patient, and the total quantity of the drug together with the daily dose and the period of consumption. A prescription that gives a dose but no total quantity does not meet the rule.

Bedside pearl. When a patient cannot get morphine, the obstacle is almost never the law as it now stands. It is that the institution never applied, or that the register and the double lock were never set up, and both are administrative problems with a named form and a 60 day clock attached to them.

11. What This Tool Does Not Cover

A decision aid cannot be read from inside for its own edges, so they are stated here.

• The opioid-naive patient. This tool converts an existing regimen. It does not set a first dose. The ISSP starting point is oral immediate release morphine 5 mg four hourly in a naive adult, 10 mg four hourly in a patient already on step 2, and 2.5 mg four to six hourly in the elderly, titrated in increments of 5 to 10 mg.
• Children. Every ratio here is adult. Paediatric palliative dosing is a separate discipline and a separate set of documents.
• Pethidine. Deliberately not converted. Its metabolite normeperidine accumulates and causes seizures, its duration is too short for chronic pain, and no dependable equianalgesic ratio for palliative use was found in the sources read. It appears in the drug list so that the tool can say this rather than stay silent when a resident selects it.
• Diamorphine. Present in every UK syringe pump table and absent from Indian practice, so the divide by three line those tables carry does not apply here. Morphine is the syringe pump opioid in India, and the subcutaneous tissue absorbs about 3 mL an hour, which becomes the practical constraint at high doses.
• Alfentanil, sufentanil and remifentanil. Not converted, for the reason given in pearl 8.
• Intrathecal and epidural routes. A different order of ratio entirely and a specialist decision.
• Opioid use disorder. Buprenorphine and methadone in substitution therapy are a different indication with different doses and a different legal route. Nothing here applies to it.
• Whether an opioid is the right answer. Neuropathic pain, bone pain, bowel colic and total pain each need something added rather than a larger opioid dose. A conversion tool cannot tell you that the pain is not opioid-responsive.