Oxycodone is Stronger Than Morphine
Oxycodone is approximately 1.5-2 times more potent than morphine when comparing equivalent oral doses, meaning 10 mg of oral oxycodone provides roughly the same analgesic effect as 15-20 mg of oral morphine. 1, 2, 3
Relative Potency Evidence
The equianalgesic conversion ratio is consistently documented across multiple high-quality guidelines:
Oral oxycodone has a relative potency of 1.5-2 compared to oral morphine (meaning oxycodone is stronger), as established by the European Society for Medical Oncology 1 and confirmed by the American College of Chest Physicians 1
This translates to a practical conversion: 10 mg oxycodone = 15-20 mg morphine 2, 3
Research studies confirm this ratio, with controlled trials demonstrating oxycodone/morphine potency ratios averaging 1:3 when doses are equated on objective measures like pupil constriction 4
Clinical Implications of Potency Difference
The higher potency of oxycodone creates specific clinical considerations:
Risk of overdosing is higher with oxycodone due to variable potency estimates, which can lead to inappropriate dose conversions 5
Oxycodone has faster onset of analgesia following both IV and oral administration compared to morphine, which may contribute to higher addiction potential 5
Despite being stronger, oxycodone and morphine demonstrate similar overall analgesic efficacy when properly dose-adjusted, with no difference in pain control or adverse effects in systematic reviews 1
Why Morphine Remains First-Line Despite Lower Potency
Morphine is recommended as the gold standard first-choice opioid for moderate to severe cancer pain by multiple guidelines 1, 2, not because it's stronger, but because:
- It is less expensive and on the WHO essential drug list 2
- It has more predictable pharmacokinetics for dose titration 1
- There is more extensive clinical experience and safety data 1
Practical Dosing Algorithm
When converting between these opioids:
Morphine to oxycodone: Divide morphine dose by 1.5-2 (e.g., 30 mg morphine → 15-20 mg oxycodone) 1, 2
Starting doses for opioid-naïve patients:
Parenteral morphine conversion: When switching from oral to IV/subcutaneous morphine, divide the oral dose by 3 to achieve equianalgesic effect 1
Critical Safety Caveat
Both opioids require identical safety precautions regardless of potency differences, including routine prophylactic laxatives for constipation 2, 3, assessment of addiction risk before initiation 2, and avoidance of benzodiazepine coprescription 2