Source Opioid Dose (mg)

Route

Fentanyl Conversion Ratio

Faculty of Pain Medicine Opioids Aware (conservative)

Reference opioid. Oral:SC/IV ratio = 2:1.

Oral Morphine Equivalent (24h)

30 mg/24h

Lower risk

Codeine (Oral)

300 mg

Poor metabolisers (~7-10% Caucasians) get minimal analgesia

Ultra-rapid metabolisers at risk of toxicity

Dihydrocodeine (Oral)

300 mg

Tramadol (Oral)

300 mg

Risk of serotonin syndrome with SSRIs/SNRIs

Lowers seizure threshold

Morphine (SC)

15 mg

Morphine (IV)

15 mg

Oxycodone (Oral)

20 mg

Oxycodone (SC)

10 mg

Oxycodone (IV)

10 mg

Tapentadol (Oral)

75 mg

Conversion ratio is an estimate; noradrenergic component not captured by OME

Hydromorphone (Oral)

6 mg

Hydromorphone (SC)

3 mg

Diamorphine (SC)

10 mg

Diamorphine (IV)

10 mg

Alfentanil (SC)

1 mg

Fentanyl (transdermal patch) (TD)

8.33 mcg/h

Conversion ratio is debated (FPM vs BNF)

Not suitable for acute/unstable pain

Takes 12-24h to reach steady state after application

Buprenorphine (BuTrans patch) (TD)

12.5 mcg/h

Partial agonist - conversion unreliable above 120mg/24h oral morphine equivalent

Specialist advice recommended for high doses

Methadone

Oral

Specialist conversion required

NEVER use a fixed conversion ratio

Always requires specialist palliative care or pain medicine input

Risk of accumulation and delayed respiratory depression

When switching opioids: reduce calculated dose by 25-50% for incomplete cross-tolerance. Titrate with breakthrough medication.