Opioid Conversion: The Evidence Behind the Calculator

A 58-year-old man with carcinoma of the oesophagus is on the palliative care ward. For a month he has taken sustained-release morphine, 60 mg twice a day, and yesterday he needed four rescue doses of 20 mg of immediate-release morphine. Since this morning he cannot swallow even water. The resident on call wants to put on a fentanyl patch and asks you what strength to write. This is an illustrative case, not a real patient.

The Opioid Conversion Calculator will do the arithmetic for him. It converts every opioid the patient takes into one figure, converts that figure out to the drug and route you want, takes off what cross-tolerance asks, and names the preparation an Indian hospital stocks. What it no longer carries is some of the why: the source behind each ratio, and the pharmacology that makes two of the drugs behave differently from the rest. Those have moved here, so that the tool stays quick at the bedside. By the end of this article you will know where each number in the tool comes from, and which disagreements between the sources can hurt your patient.

1. One currency: the oral morphine equivalent

Start with the total. Our patient takes 120 mg of sustained-release morphine and took 80 mg in rescues, so his oral morphine equivalent daily dose, the amount of oral morphine that gives the same analgesia as everything he actually took in 24 hours, is 200 mg. Notice that the rescues are counted. Leaving them out is the commonest conversion error on a ward, and it under-doses the patient on the very day he changes over.

“Why go through morphine at all? Why not convert straight from one drug to the other?” Because one common currency avoids a table of every drug pair and an answer that changes with the order you work in. A patient on a patch, a tablet and a few tramadol capsules is on three drugs and one opioid load. Convert each into oral morphine, add them, then convert the total out. You multiply the 24 hour dose by the factor to reach oral morphine, and divide by it to go the other way.

Here is every factor the tool uses, with the document each came from and where other documents differ. This is the table as the tool carried it on 4 November 2026. The tool still shows the factors and the disagreements; the Source column now lives here.

Two rows deserve a second look. Oral oxycodone: the factor used here is 1.5, but the OxyContin labels say 2, and on 120 mg of morphine that is the difference between 80 mg and 60 mg of oxycodone. Oral hydromorphone: 4 in one label and 7.5 in a systematic review. The direction you convert in decides which disagreement hurts. Coming off hydromorphone, the wider factor nearly doubles the morphine you calculate; going on to it, the same spread halves the dose. Where two conventions exist, the tool prints both and uses the one an Indian guideline states.

2. Back to our patient: a change of route is not a switch

“So the patch: 200 divided by 2.4 is about 83 mcg/h. Then take off a quarter for cross-tolerance?” Stop there for a moment. Two things are wrong with reaching for a patch today. First, a fentanyl patch takes 12 to 24 hours before its serum level settles, and up to six days at a new strength. He cannot swallow his morphine now, so a patch alone leaves him in pain tonight. Second, the patch is a new molecule, and he does not need one.

What he needs is the same drug by another route. Oral morphine to subcutaneous morphine through a syringe pump is a bioavailability correction, and the factor of 2 already contains it: 200 mg orally is 100 mg subcutaneously over 24 hours. No reduction for cross-tolerance, because the molecule has not changed. Taking a further quarter off would leave a dying man in pain on the day the pump goes up. The tool applies no reduction when only the route changes, and says so in the result.

The common mistake here is a kind one: reducing “to be safe”. It is easy to see why. Every conversion you have been taught ends with “now reduce”. This is the one place where you do not.

3. Why buprenorphine sits apart

Buprenorphine does not behave like morphine, and the reason is at the receptor. It is a partial agonist at the mu receptor with very high affinity and slow dissociation, which buys a favourable respiratory profile and a long duration. A partial agonist with a ceiling effect does not sit on a full agonist’s linear scale, which is why the CDC leaves it out of its morphine equivalent table.

Think of a tap that can only ever open halfway. Turning it further does nothing more. Morphine is a tap that keeps opening. A table that assumes every tap opens fully will mislead you about the one that cannot. That is why the published ratios for the buprenorphine patch disagree (75:1 against 100:1), and why the tool treats buprenorphine as a step 2 to early step 3 option in India, at the strengths marketed here, rather than a way to manage a large opioid requirement.

4. Methadone: why there is no single ratio

“Can’t you just give me the methadone ratio?” No, and the reason is pharmacology. Methadone is also an antagonist at the NMDA receptor, the receptor through which tolerance to a full agonist builds. NMDA antagonism is why the ratio rises with the dose already being taken instead of staying fixed. The more morphine a patient was on, the more potent methadone becomes by comparison. So the tool uses the three ISSP 2020 bands, and every source it read says methadone is started by somebody experienced with it.

Methadone also lengthens the QT interval. The S-enantiomer binds the hERG channel. That is the potassium channel whose block delays repolarisation and invites torsades de pointes. It is why the tool prints two QTc thresholds before a first dose, and why a baseline ECG is part of starting the drug.

5. The law, and what the 2014 amendment changed

Many residents believe morphine is hard to get because the law forbids it. The law changed. The Narcotic Drugs and Psychotropic Substances Act 1985 was amended in 2014 to create the Essential Narcotic Drug, a narcotic drug notified by the Central Government for medical and scientific use. It replaced a patchwork of state licences with one central rule set, and it is the single most important change to opioid access in Indian palliative care.

What the tool keeps for the bedside is the practical part: which drugs are on the notified list, that the institution and not the individual doctor holds the recognition, and what a morphine prescription must carry, including the total quantity. A prescription that gives a dose but no total quantity does not meet the rule.

Back to the ward

Our patient went on to a syringe pump with 100 mg of subcutaneous morphine over 24 hours, and a subcutaneous rescue of one sixth of that daily dose. The patch was not started that night. If he settles and the team still wants a patch later, it will be a true switch, the cross-tolerance reduction will apply, and the opioid he is already on will continue for 12 hours after the patch goes on, as the ISSP guideline advises for the four-hourly dose. Each of those steps is a line in the tool.

Try this

A 66-year-old woman with bone metastases takes tramadol 100 mg four times a day, and her pain is no longer controlled. You decide to move her to oral immediate-release morphine. What is her oral morphine equivalent, and roughly what do you start? Illustrative case.

Answer. Tramadol 400 mg a day multiplied by the oral tramadol factor of 0.2 is 80 mg of oral morphine a day. This is a change of molecule, so cross-tolerance applies: 25 to 50 per cent off gives 40 to 60 mg a day, for example 10 mg four-hourly at the lower reduction, with a rescue of one sixth of the daily dose. Because the switch is for uncontrolled pain, reduce at the lower end and titrate; the evidence is that the second opioid often needs more, not less. Then add a laxative in the same sitting. And think about bone pain itself: it often needs something added rather than a larger opioid dose.

Take-home points

• Count the rescue doses into the 24 hour total before you convert anything.
• Convert through oral morphine: multiply in, add, divide out.
• A change of route in the same molecule needs no cross-tolerance reduction; a change of molecule does.
• Buprenorphine and methadone do not sit on morphine’s straight line; start methadone only with someone experienced, and check the QTc.
• Where the published ratios disagree, know which one you are using and why; the tool prints both.

Read with this: Death and Life: A Physician’s Guide to a Good Death, for the care around the prescription.

Declaration: the author built and maintains the tool. It is free to use. The figures in this article are those the tool carried in its Evidence tab on 4 November 2026, and each factor’s source is as the tool named it. The illustrative arithmetic follows the tool’s own factors and rules; check the tool’s result for a real patient.

References

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10. Dowell D, Ragan KR, Jones CM, Baldwin GT, Chou R. CDC clinical practice guideline for prescribing opioids for pain: United States, 2022. MMWR Recomm Rep. 2022;71(3):1-95.
11. Janssen Pharmaceuticals. DURAGESIC (fentanyl transdermal system) prescribing information, revised March 2021; and Durogesic DTrans transdermal patch, summary of product characteristics, revised 19 May 2025.
12. Faculty of Pain Medicine of the Royal College of Anaesthetists. Opioids Aware: dose equivalents and changing opioids. Reviewed August 2020.
13. National Institute for Health and Care Excellence. Palliative care for adults: strong opioids for pain relief. Clinical guideline CG140. London: NICE; 2012, updated 2016.
14. Ministry of Health and Family Welfare, Government of India. National List of Essential Medicines 2022. And: The Narcotic Drugs and Psychotropic Substances Act, 1985, as amended by Act 16 of 2014, with rules 52A, 52G, 52N and 52-O of the Narcotic Drugs and Psychotropic Substances Rules, 1985.