Opioid Potency Ratios and Conversion Guidelines in Pain Management

Morphine : Oxycodone oral potency ratio

1.5 : 1 (Oxycodone is ~1.5× stronger than oral morphine.)

Morphine : Hydromorphone oral potency ratio

5 : 1 (Hydromorphone is ~5× stronger than oral morphine.)

Oral : Subcutaneous morphine conversion

2 : 1 (Oral dose ÷ 2 = subcutaneous dose.)

Oral morphine equivalent to fentanyl 25 mcg/hr patch

≈ 100 mg oral morphine per 24 h.

Cross-tolerance reduction rule

When switching opioids, reduce the calculated equianalgesic dose by 25-50 % (AMH Practice Points).

Breakthrough dose rule

1/6 of total 24-hour opioid dose given every 4 hours PRN (AMH Practice Points).

AMH advice on rounding doses

Round down to the nearest available strength to avoid overdose; titrate if needed.

AMH monitoring points for opioids

Check pain control, sedation, nausea, constipation, respiratory rate, and renal function.

Key AMH counselling point for all opioids

'Do not stop suddenly; take regularly for ongoing pain; manage constipation with laxatives.'

Mnemonic for ratios ('1.5 → 5 → 2' rule)

Morphine→Oxycodone = ÷1.5; Morphine→Hydromorphone = ÷5; Oral→Subcutaneous = ÷2.

Why reduce dose when rotating opioids?

To prevent over-sedation or respiratory depression due to incomplete cross-tolerance.