Key Facts

Opioid conversion is a high-risk prescribing activity. Errors in conversion cause significant harm. Always use conversion tables, show calculations, and seek advice if unsure.

Epidemiology

• Opioid rotation is needed in approximately 20-30% of patients on long-term opioids
• Common reasons: side effects (nausea, confusion, myoclonus), renal impairment (accumulation of metabolites), route change (inability to swallow)

Aetiology

Indications for opioid conversion:

• Route change: oral to subcutaneous/transdermal (swallowing difficulty, bowel obstruction, dying patient)
• Opioid rotation: intolerable side effects despite dose titration, poor analgesia despite dose escalation, renal impairment (morphine → oxycodone or fentanyl), development of opioid-induced hyperalgesia
• Formulation change: immediate-release to modified-release

Pathophysiology

• Incomplete cross-tolerance: different opioids act on different μ-receptor subtypes and have different receptor binding profiles; tolerance to one opioid does not fully transfer to another
• This is why dose reduction (25-50%) is needed when switching opioids
• Renal impairment: morphine-6-glucuronide (active metabolite) accumulates → toxicity; switch to oxycodone, fentanyl, or buprenorphine (hepatically metabolised)
• Fentanyl: highly lipophilic; onset and offset depend on tissue distribution; requires stable pain (not for titration)

When to Convert

• Patient unable to swallow (convert oral → SC or transdermal)
• Intolerable opioid side effects (rotate to different opioid)
• Renal impairment (switch from morphine to safer alternative)
• Starting syringe driver (convert oral to CSCI)
• Stepping up from weak to strong opioid

Assessment Before Conversion

• Current opioid, dose, route, frequency
• Total 24-hour opioid consumption (including breakthrough doses used)
• Pain control (adequate or inadequate?)
• Side effects
• Renal and hepatic function
• Reason for conversion

Red Flags

• Prescriber unsure of conversion ratios (seek specialist advice)
• Large dose conversions (high risk of error)
• Renal impairment + morphine (accumulation risk)
• Patient on mixed opioid agonists/partial agonists (buprenorphine + morphine — interactions)

Bedside

• Current pain score
• Current opioid dose (include all breakthrough doses used in last 24-48h)
• Calculate total 24-hour morphine equivalent dose
• Check renal function and hepatic function

Bloods

• U&Es (essential — renal function affects opioid choice)
• LFTs (hepatic function affects metabolism)

Imaging

• Not applicable to conversion itself

Special Tests

• Conversion calculation (show working, document clearly)
• Double-check with pharmacist or palliative care team

Key Conversion Ratios

• Oral morphine → SC/IV morphine: ÷ 2 (e.g. 60mg PO = 30mg SC)
• Oral morphine → SC/IV diamorphine: ÷ 3 (e.g. 60mg PO = 20mg SC diamorphine)
• Oral morphine → oral oxycodone: ÷ 1.5-2 (e.g. 30mg morphine ≈ 15-20mg oxycodone)
• Oral oxycodone → SC oxycodone: ÷ 1.5-2 (e.g. 20mg PO = 10-13mg SC)
• Oral morphine → fentanyl patch: use conversion table (60-90mg/24h oral morphine = 25mcg/h patch)
• Oral morphine → oral hydromorphone: ÷ 5 (e.g. 50mg morphine ≈ 10mg hydromorphone)

Conversion Steps

1. Calculate total 24-hour oral morphine equivalent (include MR + IR + breakthrough doses used)
2. Apply conversion ratio to target opioid
3. If rotating opioids: reduce by 25-50% for incomplete cross-tolerance
4. Calculate new breakthrough dose (1/6th of new 24h total)
5. Document calculation clearly in notes and prescription
6. Monitor closely after conversion (pain, sedation, respiratory rate)

Pharmacological

• Fentanyl patch: apply when starting; takes 12-24 hours to reach therapeutic levels; continue oral opioid for first 12 hours
• Syringe driver (CSCI): convert oral dose to SC diamorphine (÷3) or SC morphine (÷2); run over 24 hours
• Renal impairment: avoid morphine; use oxycodone (÷1.5-2 from morphine), fentanyl, or alfentanil in CSCI

Surgical/Interventional

• Not applicable

Referral Criteria

• Complex conversions: palliative care pharmacist or specialist
• High-dose opioid conversions (>300mg oral morphine equivalent/day): specialist mandatory
• Methadone conversion: specialist only (complex, variable half-life)
• Mixed agonist/antagonist issues: specialist advice

• Safe opioid conversion maintains pain control while reducing side effects
• Errors in conversion can cause significant harm (overdose or under-dosing)
• Most conversions are straightforward with appropriate use of conversion tables
• Close monitoring in first 24-48 hours after conversion is essential

Opioid Equianalgesic Conversion Table

Opioid Safety in Renal Impairment