Opioid Equianalgesic Conversion Calculator
Convert between opioid medications using equianalgesic dose ratios. All conversions reference oral morphine equivalents (OME). Apply 25-50% dose reduction for incomplete cross-tolerance when rotating opioids.
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Evidence-based calculations Used in clinical settings worldwide Regular monitoring recommended
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For informational purposes only — not medical advice. Consult a healthcare professional before acting on results.
🏥 Health Facts
— WHO
— CDC
What is Equianalgesic Opioid Conversion?
Equianalgesic conversion calculates equivalent doses between different opioid medications to achieve similar pain relief. This is essential when switching opioids (opioid rotation) due to intolerance, side effects, or inadequate pain control. Cross-tolerance reduction (25-50%) is critical to prevent accidental overdose.
Cross-Tolerance
Patients develop tolerance to their current opioid but have incomplete cross-tolerance to a new opioid.
Safety Rule:
- Always reduce calculated dose
- 25-50% reduction typical
- Higher reduction if switching from high doses
Route Conversion
Oral to IV ratios vary by opioid. Morphine 3:1 oral:IV, hydromorphone 5:1.
Examples:
- 30mg PO morphine = 10mg IV
- Oxycodone: IV not common
- Fentanyl: Different units (mcg)
Methadone Caution
Methadone conversion is complex due to variable half-life and NMDA receptor activity.
Special Rules:
- Higher morphine = lower ratio
- Consult specialist
- Long half-life = delayed toxicity
How Opioid Conversion Works
🔬 Conversion Process
Step-by-Step
- 1Calculate morphine equivalent of current opioid
- 2Convert morphine equivalent to new opioid
- 3Adjust for route change if needed
- 4Apply 25-50% cross-tolerance reduction
Safety Considerations
- Ratios are approximations - response varies
- Monitor closely for 24-72 hours after switch
- Renal/hepatic impairment affects dosing
- Have breakthrough medication available
When to Use Opioid Rotation
Opioid rotation (switching from one opioid to another) is a common strategy in pain management to improve analgesia or reduce side effects. Understanding when to rotate opioids is crucial for optimal patient outcomes and safety.
Side Effects
Intolerable side effects with current opioid that persist despite management attempts.
Common Issues:
- Persistent nausea/vomiting
- Severe sedation or confusion
- Intractable constipation
- Myoclonus or seizures
Inadequate Analgesia
Poor pain control despite appropriate dose escalation, suggesting tolerance.
Indicators:
- Escalating doses without benefit
- Ceiling effect reached
- Opioid-induced hyperalgesia
- Rapidly progressive pain
Route Change
Converting between administration routes based on clinical needs.
Scenarios:
- IV to oral at discharge
- Oral to IV when NPO
- Transdermal to oral
- SC to oral in hospice
Special Population Considerations
Certain patient populations require modified approaches to opioid conversion due to altered pharmacokinetics, pharmacodynamics, or increased vulnerability to adverse effects.
Elderly Patients (>65 years)
- Start at 25-50% of calculated dose
- Reduced hepatic and renal clearance
- Increased sensitivity to CNS effects
- Higher fall and fracture risk
- Extend dosing intervals if needed
Renal Impairment
- Avoid morphine - active metabolite accumulation (M6G)
- Prefer hydromorphone or fentanyl
- Reduce dose by 25-75% based on GFR
- Avoid codeine and tramadol
- Monitor for delayed toxicity
Hepatic Impairment
- Most opioids metabolized hepatically
- Reduce dose 50% in moderate impairment
- Avoid methadone (unpredictable)
- Prefer morphine (glucuronidation preserved)
- Extend dosing intervals
Opioid-Naive Patients
- No tolerance - conversion calculator not applicable
- Start low (5-10 mg morphine equivalent)
- Titrate slowly based on response
- Avoid extended-release initially
- Use immediate-release for titration
Methadone Conversion Guidelines
Methadone conversion is uniquely complex due to its variable half-life (15-60 hours), NMDA receptor antagonism, and non-linear conversion ratios. The morphine-to-methadone ratio changes significantly as morphine doses increase.
Critical Warning
Methadone has a disproportionately long elimination half-life compared to its analgesic duration. This leads to drug accumulation and delayed toxicity. Deaths have occurred 3-5 days after initiation due to respiratory depression.
Variable Conversion Ratios
| Daily Morphine Eq. | Ratio (Morphine:Methadone) |
|---|---|
| <30 mg | 2:1 |
| 30-99 mg | 4:1 |
| 100-299 mg | 8:1 |
| 300-499 mg | 12:1 |
| 500-999 mg | 15:1 |
| >1000 mg | 20:1 |
Safety Principles
- • Always consult pain/palliative specialist
- • Start at significantly lower dose than calculated
- • Maximum initial dose: 30-40 mg/day
- • Wait 5-7 days before dose increases
- • Monitor for QTc prolongation
- • Daily monitoring first 3-5 days
Post-Conversion Monitoring
Close monitoring is essential after opioid rotation to ensure adequate analgesia while preventing toxicity. The monitoring intensity depends on the complexity of the conversion and patient risk factors.
📋 Monitoring Checklist
First 24-72 Hours
- Respiratory rate q4h
- Sedation level assessment
- Pain scores regularly
- Pupil size and reactivity
Signs of Overdose
- RR <8/min
- Difficulty arousing
- Pinpoint pupils
- Cyanosis
Signs of Withdrawal
- Increased pain despite dose
- Diaphoresis, piloerection
- Nausea, diarrhea, cramps
- Restlessness, anxiety
Important Drug Interactions
Opioids interact with many medications through pharmacokinetic (CYP450) and pharmacodynamic mechanisms. These interactions can significantly affect opioid conversion calculations.
⚠️ CNS Depression Synergy
Concurrent use increases respiratory depression risk:
- • Benzodiazepines - major FDA warning
- • Gabapentinoids - pregabalin, gabapentin
- • Sedatives - Z-drugs, barbiturates
- • Alcohol - unpredictable effects
- • Muscle relaxants - additive sedation
- • Antipsychotics - hypotension risk
⚡ CYP450 Interactions
Metabolism altered by enzyme inhibitors/inducers:
- • CYP3A4 inhibitors (azoles, macrolides) increase fentanyl/methadone
- • CYP2D6 inhibitors (fluoxetine, paroxetine) block codeine → morphine
- • CYP3A4 inducers (rifampin, phenytoin) decrease methadone levels
- • Grapefruit juice - increase some opioid levels
Fentanyl Transdermal Patch Conversion
Transdermal fentanyl patches provide 72-hour continuous drug delivery. Conversion requires special consideration due to the unique pharmacokinetics of the transdermal route.
Morphine to Fentanyl Patch
| Daily Oral Morphine | Fentanyl Patch |
|---|---|
| 60-89 mg | 25 mcg/hr |
| 90-149 mg | 50 mcg/hr |
| 150-209 mg | 75 mcg/hr |
| 210-269 mg | 100 mcg/hr |
| 270-329 mg | 125 mcg/hr |
| 330-389 mg | 150 mcg/hr |
Key Considerations
- 12-24 hour delay to peak effect
- Continue previous opioid for 12-18 hours after patch application
- Heat exposure increases absorption - avoid heating pads, hot baths
- Fever increases absorption by 30%
- Cachexia affects absorption - consider lower doses
Complete Equianalgesic Reference Table
This table shows doses of each opioid equivalent to 30 mg of oral morphine. Use as a reference when calculating conversions between opioids.
| Opioid | Oral Equivalent* | Half-Life | Clinical Notes |
|---|---|---|---|
| Morphine | 30 mg | 2-4 hours | Reference standard |
| Oxycodone | 20 mg | 3-4 hours | Higher oral bioavailability |
| Hydrocodone | 30 mg | 3.8 hours | Oral only |
| Hydromorphone | 7.5 mg | 2-3 hours | More potent |
| Fentanyl | 0.1 mcg | 3-4 hours IV | mcg equivalent |
| Methadone | 4 mg | 15-60 hours | Complex conversion - variable |
| Codeine | 200 mg | 2.5-3 hours | Prodrug - variable metabolism |
| Tramadol | 300 mg | 6-7 hours | Weak opioid agonist |
| Oxymorphone | 10 mg | 7-9 hours | High potency oral |
| Tapentadol | 75 mg | 4 hours | Dual mechanism |
*Equivalent to 30 mg oral morphine. Fentanyl shown in mcg (transmucosal/parenteral).
Clinical Pearls for Opioid Conversion
Start Low, Go Slow
Always err on the side of caution. Underdosing causes temporary pain; overdosing can be fatal.
Breakthrough Availability
Always provide short-acting breakthrough medication (10-15% of 24h dose) during rotation.
Document Everything
Record rationale, calculations, and expected timeline for dose adjustments.
Patient Education
Explain what to expect during rotation - possible temporary under/over treatment.
Emergency Plan
Ensure patient/caregivers know signs of overdose and have access to naloxone.
Follow-Up
Schedule close follow-up within 1-3 days after rotation for dose titration.
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