What is the equivalent dose of immediate-release oxycodone (OxyContin) for a patient taking morphine (MS Contin) 15 mg three times a day (TID)?

Opioid Conversion: Morphine to Immediate-Release Oxycodone

For a patient taking morphine 15 mg TID (45 mg total daily), the equivalent immediate-release oxycodone dose is 20-25 mg total daily, divided as 7.5 mg TID or 10 mg TID, using the standard morphine-to-oxycodone conversion ratio of 1.5:1 to 2:1 with a 25% dose reduction for incomplete cross-tolerance. 1

Conversion Calculation

Step 1: Calculate total daily morphine dose

• Current regimen: 15 mg TID = 45 mg total daily morphine 2

Step 2: Apply conversion ratio

• The morphine-to-oxycodone conversion ratio is 1.5:1 to 2:1, meaning oxycodone is approximately 1.5-2 times more potent than morphine 1, 3
• 45 mg morphine ÷ 1.5 = 30 mg oxycodone equivalent 1

Step 3: Reduce for incomplete cross-tolerance

• Reduce the calculated dose by 25-50% to account for incomplete cross-tolerance between opioids 1
• 30 mg × 0.75 (25% reduction) = 22.5 mg oxycodone daily 1
• Conservative approach: Round to 20-25 mg total daily 1

Step 4: Divide into TID dosing

• Recommended: Immediate-release oxycodone 7.5 mg TID (22.5 mg daily) 2, 3
• Alternative: 10 mg TID (30 mg daily) if pain control inadequate 2

Clinical Implementation

Immediate-release oxycodone dosing schedule:

• Administer every 4 hours around-the-clock for baseline pain control 2
• Duration of analgesia for immediate-release formulations is approximately 4 hours 2
• Provide the same dose (7.5-10 mg) for breakthrough pain, available as often as hourly if needed 2

Titration strategy:

• Reassess pain control within 24 hours, as steady state is reached quickly with oxycodone (within 24 hours vs. 2-7 days for morphine) 3
• If more than 4 rescue doses per day are required, increase the baseline scheduled dose 2
• If pain returns consistently before the next scheduled dose, increase the regular dose rather than frequency 2

Important Safety Considerations

Monitoring requirements:

• Closely monitor for respiratory depression during the first 24-48 hours after conversion 1
• Oxycodone has more predictable metabolism than morphine, making titration easier 3
• Oral bioavailability of oxycodone is 60-87% (higher than morphine's ~30%), contributing to its greater potency 3

Renal impairment caveat:

• In significant renal impairment (eGFR <30 mL/min), oxycodone may be preferred over morphine due to less accumulation of active metabolites 2, 1
• Morphine-6-glucuronide accumulates in renal insufficiency and can worsen adverse effects 2

Individual variability:

• The conversion ratio is unpredictable in clinical practice due to asymmetric tolerance and different receptor affinities 1
• Conservative initial dosing is mandatory—starting at the lower end of the calculated range (7.5 mg TID) is safer 1, 4

Common Pitfalls to Avoid

• Do not use a 1:1 conversion ratio—this will result in overdosing, as oxycodone is significantly more potent than morphine 1, 3
• Do not skip the cross-tolerance reduction—always reduce by 25-50% when switching opioids to prevent toxicity 1
• Do not increase dosing frequency beyond every 4 hours—instead, increase the dose itself for better compliance and simpler regimens 2
• Avoid intramuscular administration—subcutaneous is simpler and less painful if parenteral route becomes necessary 2