Pain Equivalent Chart for Opioid Rotation and Conversion
Pain equivalent charts (opioid equianalgesic conversion tables) are essential clinical tools for opioid rotation, but they must be used with caution—always reduce the calculated equianalgesic dose by 25-50% when switching opioids to account for incomplete cross-tolerance and individual variability. 1
Core Principle: Why Dose Reduction is Critical
Opioid rotation improves the analgesia/toxicity relationship by using lower doses than predicted by conversion tables, which reduces opioid toxicity while maintaining or improving pain control in patients with inadequate analgesia or intolerable side effects. 1
Individual patients vary greatly in their response to different opioids due to asymmetric tolerance, different receptor affinities, pharmacokinetic profiles, and genetic factors—making strict adherence to conversion ratios potentially dangerous. 1
10-30% of patients on oral morphine fail to achieve adequate outcomes due to excessive adverse effects, inadequate analgesia, or both, making opioid rotation a necessary clinical strategy. 1
Standard Conversion Approach for Most Opioids
For Morphine, Oxycodone, and Hydromorphone
Calculate the total 24-hour opioid requirement in oral morphine equivalents, then convert to the new opioid using standard ratios, but reduce the calculated dose by 25-50% to account for incomplete cross-tolerance. 1
These three opioids are more manageable in clinical practice compared to methadone due to more predictable conversion ratios. 1
For opioid-tolerant patients in acute pain crisis, administer 10-20% of the previous 24-hour total oral morphine equivalent as the initial rescue dose. 1, 2
Reassessment Timeline
For oral administration: reassess at 60 minutes; if pain unchanged, increase dose by 50-100% after 2-3 cycles. 1, 2
For intravenous administration: reassess at 15 minutes; if pain unchanged, increase dose by 50-100% after 2-3 cycles. 1, 2
Special Case: Methadone Conversion
Methadone requires dose-dependent conversion ratios that are inversely proportional to the previous morphine dose—higher morphine doses require more conservative methadone ratios. 1
Methadone-Specific Ratios
For patients taking <90 mg oral morphine daily: use a 4:1 ratio (morphine:methadone). 1
For patients taking 90-300 mg oral morphine daily: use an 8:1 ratio. 1
For patients taking >300 mg oral morphine daily: use a 12:1 ratio or higher. 1
Critical Methadone Considerations
Methadone has a slow onset due to large volume of distribution and requires a priming dose, with accumulation occurring after 2-3 days rather than immediately. 1
A starting dose of 1/5 of the previous morphine dose may be appropriate as a loading dose, to be adjusted in subsequent days based on response and accumulation. 1
Methadone rotation carries higher risk in patients on very high morphine doses, those highly tolerant, or those on long-term therapy—these patients require closer monitoring. 1
Practical Implementation Algorithm
Step 1: Calculate Total Daily Opioid Exposure
Sum all scheduled and as-needed opioid doses taken in the previous 24 hours. 1
Convert to oral morphine equivalents using standard conversion factors. 1, 2
Step 2: Apply Appropriate Conversion Ratio
For morphine/oxycodone/hydromorphone: reduce calculated dose by 25-50%. 1
For methadone: use dose-dependent ratios (4:1,8:1, or 12:1) based on previous morphine dose. 1
Step 3: Divide into Scheduled and Rescue Doses
Provide 70-80% of total daily dose as scheduled (around-the-clock) medication. 1
Provide 10-20% of total daily dose as rescue doses for breakthrough pain, available every 1-2 hours for short-acting formulations. 1, 2
Step 4: Titrate Based on Response
If patient requires >4 rescue doses per 24 hours, increase the scheduled baseline dose by adding the total rescue medication used to the next day's scheduled regimen. 1, 2
Reassess pain and side effects at appropriate intervals (60 minutes for oral, 15 minutes for IV). 1, 2
Common Pitfalls and How to Avoid Them
Overestimation of Cross-Tolerance
The single most dangerous error is using full equianalgesic doses without reduction—this can lead to oversedation and respiratory depression. 1
Always reduce by at least 25% when switching opioids, with 50% reduction being safer for high-risk patients. 2
Inadequate Monitoring During Methadone Conversion
Methadone accumulation occurs over 2-3 days, not immediately—patients may appear stable initially but develop toxicity on days 3-5. 1
Daily reassessment is mandatory during the first week of methadone conversion. 1
Failure to Account for Route Changes
- When converting from oral to IV, remember that IV morphine is approximately 3 times more potent than oral morphine—calculate the 24-hour IV equivalent before determining the 10-20% rescue dose. 1, 2
Ignoring Renal Function
- Morphine metabolites accumulate in renal impairment, potentially causing prolonged sedation and toxicity—consider alternative opioids (oxycodone, hydromorphone, fentanyl) in patients with significant renal dysfunction. 2
Indications for Opioid Rotation
Opioid rotation is indicated when the adverse effect/analgesic equation is skewed toward side effects despite aggressive adjuvant treatment. 1
Inadequate analgesia despite dose escalation. 1
Intolerable side effects (sedation, nausea, constipation, cognitive impairment) that persist despite management. 1
Combination of both inadequate analgesia and excessive side effects. 1
Adjunctive Measures During Rotation
Always initiate a bowel regimen when starting or rotating opioids, as constipation tolerance rarely develops. 1
Consider adding coanalgesics (antidepressants for neuropathic pain, NSAIDs for inflammatory pain) to reduce total opioid requirements. 1
Provide psychosocial support and patient/family education about the rotation process, expected timeline for improvement, and warning signs requiring immediate contact. 1, 2