Oral to Parenteral Opioid Ratio Calculator
Oral to Parenteral Opioid Ratio Calculator
The parenteral equivalent of an oral opioid dose by both published divisors at once — because oral morphine is divided by 2 in some sources and 3 in others, and because the product label’s figure for converting back does not return the dose you began with.
Oral dose as a parenteral dose, by both published divisors
4 pairs, 2 divisors, 1 broken round tripOral morphine 60 mg a day, converting to parenteral morphine
Two divisors, one bioavailability, and a round trip that does not close
Oral morphine to parenteral morphine: ÷ 2 (Scottish Palliative Care Guidelines, Pharmaceutical Journal) or ÷ 3 (MSD, 30 mg oral to 10 mg parenteral)
Oral morphine to subcutaneous diamorphine: ÷ 3
Oral oxycodone to subcutaneous oxycodone: ÷ 2
Oral morphine to subcutaneous alfentanil: ÷ 30
The other way, from the oral solution’s own label: replacing parenteral morphine with the oral solution needs “a 50% to 100% increase in dosage”, so × 1.5 to × 2
- why there is a ratio at all
- oral morphine is extensively metabolised on its first pass through the liver. Its own Summary of Product Characteristics puts the systemic bioavailability at “approximately 25%”, so about three quarters of a swallowed dose never reaches the circulation as morphine
- why the ratio is not 4
- because the metabolism that destroys three quarters of the dose also produces morphine-6-glucuronide, which the same document says has “pharmacological effects indistinguishable from those of morphine”. An oral dose therefore delivers less morphine and more active metabolite, and the clinical ratio of 2 to 3 sits well below the pharmacokinetic one
- 2 or 3
- the Scottish Palliative Care Guidelines and the Pharmaceutical Journal’s palliative care table divide oral morphine by 2; MSD’s equianalgesic table pairs 30 mg oral with 10 mg parenteral, which is 3. A 1.5-fold disagreement on the commonest conversion in palliative care
- why out and back do not cancel
- because the two directions come from different documents written for different purposes. Divide 60 mg of oral morphine by 3 and the answer is 20 mg parenteral; take that 20 mg back to oral by the label’s 50 per cent uplift and the answer is 30 mg, not 60. Neither step is wrong and the round trip still loses half the dose
- diamorphine, and why its divisor is 3
- diamorphine is more soluble than morphine, which is why it is the subcutaneous drug of choice in United Kingdom practice, and the Scottish Palliative Care Guidelines divide the oral morphine dose by 3 to reach it. That is a different claim from the oral-to-parenteral-morphine ratio and the two are not interchangeable
- renal function
- morphine-6-glucuronide is cleared by the kidney and “may accumulate in patients with renal failure”. No oral-to-parenteral ratio has a term for renal function, and the accumulation does not care which route the dose arrived by
- what this page does not render
- a dose. Both figures are arithmetic applied to a published divisor, and the published reduction for incomplete cross-tolerance still applies on top of either of them when the drug as well as the route changes
Worked example
Oral morphine 60 mg a day, converting to parenteral morphine
By the divisor of 2 used in the Scottish Palliative Care Guidelines and the Pharmaceutical Journal's table: 60 ÷ 2 = 30.00 mg a day parenterally
By MSD's divisor of 3, from 30 mg oral against 10 mg parenteral: 60 ÷ 3 = 20.00 mg a day
Larger divisor ÷ smaller = 3 ÷ 2 = 1.50-fold between two published answers to the same question
Now back the other way, using the oral solution's own label. From 30 mg parenteral, the "50% to 100% increase" yields 45.00 to 60.00 mg by mouth — the top of that range closes the round trip and the bottom does not
From 20 mg parenteral, the same uplift yields 30.00 to 40.00 mg by mouth. Out by 3 and back by 1.5 returns 30.00 against the 60 we began with: the round trip loses half the dose, and every number in it is correctly taken from a published source
Oral morphine 60 mg a day to subcutaneous diamorphine, divisor 3: 20.00 mg over 24 hours. That is a different claim from the parenteral morphine figure and the two must not be read as alternatives for the same drug
Oral oxycodone 30 mg a day to subcutaneous oxycodone, divisor 2: 15.00 mg. Reaching subcutaneous oxycodone from oral morphine instead needs two steps, and the Scottish Palliative Care Guidelines' direct divisor for that is 4 — which is their oxycodone potency of 2 multiplied by this 2, so the table is internally consistent there
Oral morphine 300 mg a day to subcutaneous alfentanil, divisor 30: 10.00 mg over 24 hours, a conversion both sources that publish it describe as needing specialist palliative care input
Published oral to parenteral divisors, with the source of each
| Conversion | Divisor | Source | Second source |
|---|---|---|---|
| Oral morphine to parenteral morphine | 2 | Scottish Palliative Care Guidelines | Pharmaceutical Journal, overview of opioids in palliative care |
| Oral morphine to parenteral morphine | 3 | MSD Manuals equianalgesic table, 30 mg oral against 10 mg parenteral | Wellington ICU Appendix 6 implies about 3 from its fentanyl rows |
| Parenteral morphine to oral morphine | × 1.5 to × 2 | Morphine oral solution Summary of Product Characteristics: replacing parenteral morphine needs “a 50% to 100% increase in dosage” | none read |
| Oral morphine to subcutaneous diamorphine | 3 | Scottish Palliative Care Guidelines | none read |
| Oral oxycodone to subcutaneous oxycodone | 2 | Scottish Palliative Care Guidelines | Pharmaceutical Journal, same table |
| Oral morphine to subcutaneous oxycodone | 4 | Scottish Palliative Care Guidelines | consistent with 2 × 2 in the same table |
| Oral morphine to subcutaneous alfentanil | 30 | Scottish Palliative Care Guidelines | Pharmaceutical Journal, same table; both say specialist input is needed |
| Oral morphine to subcutaneous hydromorphone | 10 | Scottish Palliative Care Guidelines, specialist input needed | none read |
| Oral methadone to parenteral methadone | 2 | Wellington ICU Drug Manual Appendix 6 | none read |
The reduction for incomplete cross-tolerance, by source
| Source | Reduction applied to the calculated equivalent | What else it says |
|---|---|---|
| Faculty of Pain Medicine, Opioids Aware | 25 to 50 per cent in most switches | At least 50 per cent above about 500 mg of oral morphine equivalent a day, in the elderly or frail, or where adverse effects are intolerable |
| Scottish Palliative Care Guidelines | Up to 30 per cent | “Consider reducing the dose by up to 30%” and re-titrate; reduce further if the patient is opioid toxic, frail or elderly |
| Severn Hospice conversion table | 25 to 30 per cent | 50 per cent when converting high doses, to avoid toxicity |
| MSD Manuals equianalgesic table | 50 per cent | 75 to 90 per cent for methadone, and the same reduction still applies on top of the dose-banded methadone ratios |
| Wellington ICU Drug Manual, Appendix 6 | 25 to 50 per cent | Calculate the equianalgesic figure first, then cut it |
| Fine and Portenoy, via Treillet et al. 2018 | 25 to 50 per cent, then a further 15 to 30 per cent either way | Explicitly excepts methadone and transdermal and transmucosal fentanyl from the first step |
Twenty-five per cent bioavailability, and a ratio of two or three
Oral morphine is extensively metabolised before it reaches the circulation. Its own Summary of Product Characteristics puts the systemic bioavailability at “approximately 25%” after “significant first pass metabolism in the liver”, which is why a swallowed dose and an injected one are not the same quantity of drug. That is the whole origin of the oral-to-parenteral ratio.
It is not, however, why the ratio is 2 or 3 rather than 4. The first-pass metabolism that removes three quarters of the dose also produces morphine-6-glucuronide, which the same document describes as having “pharmacological effects indistinguishable from those of morphine”. An oral dose therefore delivers less morphine and more active metabolite than the bioavailability figure alone suggests, and the clinically used ratio sits well below the pharmacokinetic one. It is also why renal impairment changes the answer in a way no ratio can express: morphine-6-glucuronide is renally cleared and “may accumulate in patients with renal failure”, so the same oral dose produces a different effect at a different estimated glomerular filtration rate.
The published divisor is 2 in the Scottish Palliative Care Guidelines and the Pharmaceutical Journal’s palliative care table, and 3 in MSD’s equianalgesic table, which pairs 30 mg of oral morphine with 10 mg parenteral. A 1.5-fold disagreement on the commonest conversion in palliative medicine is not a rounding difference, and both figures are shown here for that reason. Diamorphine, which is the subcutaneous drug of choice in United Kingdom practice because it is far more soluble, has its own divisor of 3 from oral morphine — a different claim that is easy to mistake for the parenteral morphine figure.
The direction then matters more than it looks as though it should. The morphine oral solution’s label says that when it replaces parenteral morphine, “a 50% to 100% increase in dosage is usually required in order to achieve the same level of analgesia” — a ratio of 1.5 to 2, not 2 to 3. Convert 60 mg of oral morphine out by the divisor of 3 and back by the 50 per cent uplift and you arrive at 30 mg, half of what you began with, with every step correctly taken from a published source. That is what non-reciprocity looks like in practice, and it is the same phenomenon the rotation page shows for oral morphine against oral hydromorphone. These are population approximations; the published cross-tolerance reduction applies after any equivalence; your own formulary governs. The notes below set all three out in full.
Frequently asked questions
Is oral morphine divided by 2 or by 3 to reach a parenteral dose?
Both are published. The Scottish Palliative Care Guidelines and the Pharmaceutical Journal’s palliative care table divide by 2; MSD’s equianalgesic table pairs 30 mg oral with 10 mg parenteral, which is 3. This page prints both and the 1.5-fold gap between them. Which one a service uses is a local convention, and the local chart governs.
Why does converting back not return the original dose?
Because the two directions come from different documents. The conversion charts divide oral by 2 or 3 going out; the morphine oral solution’s own label says that replacing parenteral morphine with the oral solution needs “a 50% to 100% increase in dosage”, which is a ratio of 1.5 to 2 coming back. Out by 3 and back by 1.5 loses half the dose. Nothing is miscalculated; the published ratios simply are not reciprocal.
If oral bioavailability is only 25 per cent, why is the ratio not 4?
Because the first-pass metabolism that removes most of the dose also makes morphine-6-glucuronide, which the Summary of Product Characteristics describes as having “pharmacological effects indistinguishable from those of morphine”. An oral dose delivers less morphine and more active metabolite, so the clinically used ratio of 2 to 3 is smaller than the pharmacokinetic one. It is also why renal impairment shifts the answer: that metabolite is renally cleared and accumulates when clearance falls.
Is subcutaneous diamorphine the same as parenteral morphine?
No, and the divisors differ. The Scottish Palliative Care Guidelines divide oral morphine by 2 for subcutaneous morphine and by 3 for subcutaneous diamorphine, because diamorphine is more potent per milligram as well as far more soluble — which is why it is the usual subcutaneous choice in United Kingdom practice. Reading the diamorphine figure as a morphine figure overstates the dose by half again.
Does the cross-tolerance reduction apply to a route change?
The sources apply it to a change of drug. Where the route changes and the drug does not — oral morphine to parenteral morphine — there is no new drug to be incompletely tolerant to, and the published guidance on switching does not name that case. Where both change at once, as in oral morphine to subcutaneous diamorphine or to alfentanil, the reduction applies on top of the divisor, and the table on this page lists the six published versions of it.
Related calculators
References
- Morphine 10mg/5ml Oral Solution. Summary of Product Characteristics, electronic Medicines Compendium, PL 00156/0036. Section 5.2: “Morphine undergoes significant first pass metabolism in the liver”, with a “systemic bioavailability of approximately 25%”, and “Morphine-6-glucuronide has pharmacological effects indistinguishable from those of morphine”; section 4.4 adds that it “may accumulate in patients with renal failure”. Section 4.2 is the figure this batch uses: when the oral solution replaces parenteral morphine, “a 50% to 100% increase in dosage is usually required in order to achieve the same level of analgesia” — an oral to parenteral ratio of 1.5 to 2, against the 2 and 3 the conversion charts publish.
- NHS Scotland. Scottish Palliative Care Guidelines: opioid/opiate conversion tables — switching between opioid medicines (Right Decision Service). Divisors on the 24-hour oral morphine dose: 2 subcutaneous morphine, 3 subcutaneous diamorphine, 2 oral oxycodone, 4 subcutaneous oxycodone, 5 to 7.5 oral hydromorphone, 10 subcutaneous hydromorphone, 30 subcutaneous alfentanil; codeine, dihydrocodeine and tramadol divided by 10. A seven-day buprenorphine patch at 5 micrograms an hour is matched to 12 mg of oral morphine a day. On switching: “Consider reducing the dose by up to 30%” and re-titrate, reducing further if the patient is opioid toxic, frail or elderly. Its oxycodone divisor of 2 is the figure that disagrees with the Faculty of Pain Medicine’s 1.5.
- MSD Manuals Professional Version. Equianalgesic Doses of Opioid Analgesics. Parenteral against oral: morphine 10 to 30 mg, codeine 130 to 200, hydromorphone 1.5 to 7.5, methadone 10 to 20, oxycodone 15 to 20, oxymorphone 1 to 15. Its footnotes state that the equivalences come from single-dose studies and clinical experience and are approximations; that on changing opioid the equianalgesic dose is cut by 50 per cent, and by 75 to 90 per cent for methadone; and that the morphine-to-methadone ratio is non-linear, rising from about 2:1 below 30 mg of oral morphine equivalent a day to about 20:1 at 1000 mg a day and above. Its oral oxymorphone figure of 15 mg against 30 mg of morphine implies a potency of 2, where CDC publishes 3.
- Faculty of Pain Medicine of the Royal College of Anaesthetists. Opioids Aware: dose equivalents and changing opioids. Oral potencies, reviewed March 2023 against the BNF: codeine 0.1, dihydrocodeine 0.1, hydromorphone 5, morphine 1, oxycodone 1.5, tapentadol 0.4, tramadol 0.1, methadone “varies” with specialist advice required. Transdermal fentanyl 12, 25, 50, 75 and 100 micrograms an hour against 30, 60, 120, 180 and 240 mg of oral morphine a day. Conversion factors are an approximate guide only “because data are incomplete and individual variation is significant”; in most switches the calculated equivalent is cut by 25 to 50 per cent, and by at least 50 per cent above about 500 mg of oral morphine equivalent a day or in the elderly or frail; and “Opioid rotation is not recommended if a patient has responded to one opioid”.
- Severn Hospice. Opioid Conversion Table, version 07.19. Oral potencies of 0.1 for codeine and dihydrocodeine but 0.15 for tramadol, and oral oxycodone reached by dividing oral morphine by 2 — two figures that disagree with the Faculty of Pain Medicine’s 0.1 and 1.5 for the same drugs. Transdermal fentanyl “approx. 100 to 150 times more potent than oral morphine” with the table built at 100:1; buprenorphine 5 micrograms an hour matched to 12 mg of oral morphine a day. Puts the switching reduction at 25 to 30 per cent, and at 50 per cent when converting high doses, to avoid toxicity.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/