Mastering Opioid Rotation and Conversion for MRCP PACES

Few topics create as much quiet panic in the PACES exam hall as opioid conversion. The examiner asks you to state your prescription, and suddenly every dose you ever knew evaporates. Yet opioid rotation is one of the most predictable, high-yield areas of palliative care in MRCP PACES — and the same calculations reappear in MRCP Part 1 and Part 2 best-of-five questions. Master the method once, and you bank marks across the entire diploma.

Why Examiners Love Opioid Rotation

A single opioid scenario simultaneously tests:

• Clinical pharmacology — equianalgesic doses, routes, organ impairment

• Prescribing safety — dose reduction, breakthrough prescribing, laxatives

• Guideline awareness — NICE CG140 (safe prescribing of strong opioids in palliative care)

• Numeracy under pressure — a genuine differentiator between candidates

• Communication — addressing myths about morphine, addiction and dying

Typical PACES triggers for a rotation include uncontrolled pain, opioid toxicity, dysphagia, persistent nausea, and worsening renal function. Recognising why the rotation is needed is worth as many marks as the arithmetic itself.

The Core Concept: Oral Morphine Equivalents (OME)

Everything is anchored to oral morphine per 24 hours. Convert whatever the patient takes into OME first, then convert out to the new drug. Never convert drug-to-drug directly — that is where errors happen.

The Conversion Table You Must Memorise

Critical caveat: these figures are approximations. Published sources (BNF, Palliative Care Formulary, local trust charts) vary, and examiners know it. The safe exam answer is: approximately X mg based on standard conversion tables, cross-checked against local palliative care policy, with specialist palliative team involvement if uncertain. Saying that sentence earns marks even before your arithmetic does.

Route Rules

• Oral morphine : SC morphine = 2 : 1 — halve the dose when going subcutaneous

• Oral oxycodone : SC oxycodone = 2 : 1

• Fentanyl patches take 12–24 hours to reach steady state — never use in opioid-naive patients, unstable or acute pain; keep rescue medication available during the lag

• Methadone conversions are highly variable and lipid-accumulating — specialist prescribing only. Saying this in PACES is itself a mark-scoring move

The Six-Step Rotation Method

1. Total the last 24 hours — regular opioid plus every PRN dose actually taken (candidates routinely forget PRN; this is the commonest arithmetic error)

2. Convert to OME using the table

3. Choose the new drug and route (driven by the clinical trigger — dysphagia, toxicity, renal impairment)

4. Reduce the equianalgesic dose by 25–50% for incomplete cross-tolerance. Uncontrolled pain → reduce ~25%; significant toxicity or high doses → reduce ~50%

5. Split the dose (modified-release twice daily, or a 24-hour SC infusion) and prescribe breakthrough analgesia = 1/6 of the new total daily dose, minimum interval ~4 hours orally, ~1 hour SC

6. Complete the safety bundle and review in 24–48 h — see below

Worked Examples (Practise These Aloud)

1. Stepping Up From a Weak Opioid

A patient takes co-codamol 30/500, two tablets QDS, with ongoing pain.

• Codeine 240 mg/24 h ≈ OME 30 mg

• Stop co-codamol; continue paracetamol 1 g QDS separately

• Start morphine sulfate modified-release 15 mg BD

• PRN: morphine immediate-release 5 mg (1/6 of 30)

• Add senna (± softener); short-course antiemetic if needed

2. Rotating to a Patch (Dysphagia)

MST 60 mg BD (= 120 mg OME/24 h) with nausea and swallowing difficulty.

• Target: transdermal fentanyl. Using ~25 mcg/h ≈ 60–90 mg OME, choose fentanyl 50 mcg/h patch (covers ~120–180 mg) — the banding provides a built-in safety margin

• Stop the MST when the patch goes on — duplicating long-acting opioids is a classic prescribing disaster

• PRN if swallowing fails: SC morphine 10 mg (1/6 of 120 = 20 mg oral → 10 mg SC)

• Counsel: change every 72 h, rotate sites, avoid heat (hot baths, heat pads, fever increase absorption), fold and safely dispose of used patches

3. Morphine Toxicity in Renal Impairment

eGFR 28, on MST 30 mg BD + Oramorph 10 mg PRN ×2, now drowsy with myoclonus.

• Total = 60 + 20 = 80 mg OME → this is opioid toxicity from morphine metabolite accumulation (M3G/M6G), not simply uncontrolled pain

• Stop morphine. Rotate to a renally-safer opioid: fentanyl 25 mcg/h patch (≈60–90 mg OME) — effectively a ~30% dose reduction without extra arithmetic

• PRN: cautious low-dose oxycodone oral liquid 5 mg with close review

• Recheck renal function, address triggers, involve the specialist palliative care team

4. Syringe Driver at End of Life

Patient unable to swallow, OME 60 mg/24 h.

• SC morphine 30 mg over 24 h via syringe pump (2:1 rule)

• PRN SC morphine 5 mg, repeatable after ~1 hour, maximum ~6 doses per 24 h before reassessment

• Add appropriate adjuvants; stop oral laxatives if the patient is not eating

Titration rule: if the patient consistently needs ≥3 breakthrough doses per 24 h, recalculate the total and increase the baseline (commonly by ~50% of total PRN used), then review again in 48 h.

Beyond Opioids: The Adjuvant Marks

Strong opioids alone will not carry a PACES management plan:

• Neuropathic pain (NICE CG173): offer a choice of amitriptyline, duloxetine, gabapentin or pregabalin first-line — except trigeminal neuralgia, where carbamazepine is first-line. Tramadol is rescue therapy only

• Bone pain: NSAID (with PPI cover), bisphosphonates, single-fraction radiotherapy

• Nerve compression / raised ICP / liver capsule pain: dexamethasone

• Total pain concept: explicitly acknowledge physical, psychological, social and spiritual dimensions — say it out loud, it scores

• Always pair opioids with a stimulant laxative unless the patient has diarrhoea or a stoma — omitting this is one of the commonest single mark-losers in the exam

Organ Impairment Essentials

Opioid Toxicity: Recognise and Rescue

Recognise: sedation → confusion/hallucinations → myoclonus → pinpoint pupils → respiratory depression (late). A drowsy palliative patient on opioids is toxic until proven otherwise.

Rescue: if respiratory depression, give naloxone in small titrated increments (e.g., 40–100 micrograms IV, diluted, repeated every 2 minutes) — enough to restore respiration without precipitating a pain crisis or withdrawal. Remember naloxone is shorter-acting than most opioids, especially patches: re-observe and consider an infusion.

How This Plays Out in PACES

• History-focused stations: a pain history with SOCRATES, chronological opioid history (exact doses, PRN frequency, adherence), past alcohol/opioid use, and exploration of concerns — many patients fear that starting morphine means dying. Directly addressing that belief is a guaranteed communication-domain win

• Station 5-style consultations: typically a deteriorating patient on opioids; the hidden task is recognising toxicity or a changing clinical state (new leg weakness = think spinal cord compression, one of the four palliative care emergencies)

• Expect examiner probing on: why reduce the dose on rotation (incomplete cross-tolerance), why halve for SC (bioavailability), what is your PRN dose and interval, when will you review, who will monitor at home

Model Walkthrough

68-year-old with metastatic breast cancer and bone metastases on MST 30 mg BD plus Oramorph 10 mg × 4 in the last 24 h, now drowsy with twitching; eGFR 30; daughter asks for stronger painkillers.

1. Recognise toxicity, not under-treatment (total 100 mg OME with sedation and myoclonus)

2. Explain simply: the kidneys cannot clear the morphine by-products, which is why she is drowsy

3. Plan out loud: stop oral morphine, rotate to fentanyl 25 mcg/h (≈60–90 mg band = built-in reduction), cautious PRN, stop and review all sedating drugs

4. Safety-net: review within 24 h, specialist palliative care and community nursing involvement, clear escalation plan for new neurological symptoms

5. Address the daughter directly: reassure that this is dose optimisation, not withdrawal of care

Ten Marks-Losing Pitfalls

1. Starting a strong opioid without PRN breakthrough dosing

2. No laxative prescribed

3. Forgetting to include PRN doses in the 24-hour total

4. No 25–50% dose reduction when rotating

5. Wrong oral:SC ratio (it is 2:1, not 1:1)

6. Skipping patch counselling — 72-hourly change, site rotation, heat, disposal

7. Ignoring renal function before escalating

8. Leaving a duplicate long-acting opioid after starting a patch

9. No review plan or specialist palliative care involvement

10. Burying the plan in jargon instead of checking understanding and addressing family concerns

Rapid Revision Box

• Anchor: oral morphine 30 mg/24 h

• 30 ≈ oxycodone 20 ≈ codeine 200–240 ≈ tramadol 300–400 ≈ SC morphine 15 ≈ fentanyl 12 mcg/h

• Rotate → cut 25–50%

• PRN = 1/6 of total daily dose

• Oral : SC = 2 : 1

• Always laxative + review in 24–48 h

• eGFR < 30 → fentanyl, buprenorphine or alfentanil

• NICE CG140 for safe strong-opioid prescribing

Self-Test Drills

1. Tramadol MR 100 mg TDS + paracetamol, pain still 7/10 → ?
   OME ~30 → stop tramadol, MST 15 mg BD + Oramorph 5 mg PRN + senna.

2. MST 90 mg BD + Oramorph 30 mg × 3, needing a renal-safe switch → ?
   Total 270 mg OME → oxycodone equivalent 180 mg → reduce ~30% → ~125 mg → oxycodone MR 60 mg BD + PRN oxycodone IR ~20 mg, or fentanyl patch if eGFR poor.

3. Fentanyl 75 mcg/h, now too weak to swallow → rotate to SC?
   Trick question — patches are unaffected by swallowing. Continue the patch, add SC PRN only.

Final Word

Opioid rotation is not about memorising a spreadsheet — it is about demonstrating a safe, systematic, communicative process under time pressure. Learn the six steps, rehearse the arithmetic aloud, and always finish with the safety bundle and a review plan. Practise these conversations early and often with simulated patients, because in PACES the numbers earn the pass — but the communication earns the excellence.