Opioid Conversion Calculator
Convert between opioid medications using morphine milligram equivalents (MME). Enter values for instant results with step-by-step formulas.
Formula
MME = Source Dose x Source Conversion Factor; Target Dose = MME / Target Conversion Factor x (1 - Reduction%)
The source dose is first converted to morphine milligram equivalents by multiplying by the opioid-specific conversion factor. The MME is then divided by the target opioid conversion factor to get the equianalgesic dose, which is further reduced by 25-50% to account for incomplete cross-tolerance between opioids.
Worked Examples
Example 1: Morphine to Oxycodone Conversion
Problem: A patient takes morphine 30 mg PO every 8 hours (90 mg/day) and needs conversion to oxycodone due to side effects. Apply 25% dose reduction.
Solution: Total daily morphine: 30 mg x 3 = 90 mg/day\nMME: 90 mg (morphine PO ratio = 1:1)\nOxycodone ratio: 1.5x morphine potency\nEquivalent oxycodone: 90 / 1.5 = 60 mg/day\nWith 25% reduction: 60 x 0.75 = 45 mg/day\nDivided q8h: 45 / 3 = 15 mg every 8 hours\nBreakthrough: 15% of 45 = 6.75, round to 5-7.5 mg q4-6h PRN
Result: Oxycodone 15 mg PO q8h (45 mg/day) + 5-7.5 mg PO q4-6h PRN breakthrough
Example 2: Oxycodone to Fentanyl Patch Conversion
Problem: A patient on oxycodone 40 mg PO twice daily (80 mg/day) needs conversion to fentanyl patch. Calculate patch strength with 25% reduction.
Solution: Total daily oxycodone: 80 mg/day\nMME: 80 x 1.5 = 120 mg morphine equivalents/day\nFentanyl patch conversion: 120 / 2.4 = 50 mcg/hr\nWith 25% reduction: 50 x 0.75 = 37.5 mcg/hr\nNearest available patch: 37 mcg/hr (or 25 + 12 mcg/hr)\nBreakthrough: morphine 15-20 mg PO q4h PRN during transition
Result: Fentanyl 37 mcg/hr patch q72h + morphine IR 15-20 mg PO q4h PRN
Frequently Asked Questions
How is methadone conversion different from other opioids?
Methadone conversion is uniquely complex because its potency ratio relative to morphine changes dramatically based on the total morphine equivalent dose being converted. At low morphine doses under 30 mg/day, methadone is approximately twice as potent as morphine, but at high morphine doses above 300 mg/day, methadone can be 12 to 15 times more potent. This variable ratio makes methadone conversion the most dangerous opioid rotation, with conversion-related deaths well-documented in the literature. Methadone also has an extremely long and unpredictable half-life of 8 to 59 hours, meaning toxicity can develop days after initiation. Methadone conversions should only be performed by clinicians with specific expertise, using conservative conversion ratios and slow titration over several days.
How should breakthrough pain dosing be calculated during opioid rotation?
Breakthrough pain doses are typically calculated as 10 to 20 percent of the total 24-hour opioid dose, with 15 percent being the most commonly used starting point. When converting to a new opioid, the breakthrough dose should be calculated using the new reduced total daily dose in the target opioid rather than the previous medication. For example, if a patient converts to 40 mg of oral oxycodone daily after dose reduction, the breakthrough dose would be approximately 5 to 8 mg of immediate-release oxycodone every 4 to 6 hours as needed. The frequency of breakthrough dosing should be monitored closely because using more than 3 to 4 breakthrough doses per day suggests the baseline dose needs adjustment. During the transition period, breakthrough use patterns help guide dose titration of the new medication.
How do CYP enzyme polymorphisms affect opioid conversion accuracy?
Cytochrome P450 enzyme polymorphisms significantly impact opioid metabolism and can make standard equianalgesic conversions inaccurate for individual patients. Codeine is a prodrug that requires CYP2D6 activation to morphine, and approximately 7 to 10 percent of Caucasians are poor metabolizers who get essentially no analgesic effect from codeine. Conversely, ultra-rapid CYP2D6 metabolizers convert codeine to morphine too quickly, risking toxicity. Tramadol is similarly dependent on CYP2D6 for conversion to its active metabolite O-desmethyltramadol. Oxycodone metabolism through CYP2D6 produces oxymorphone, contributing to its analgesic effect. Methadone is primarily metabolized by CYP3A4, CYP2B6, and CYP2D6, making it susceptible to numerous drug interactions. Pharmacogenomic testing can help predict metabolism patterns and guide safer dosing.
How should opioid rotation be managed in patients with renal impairment?
Renal impairment significantly affects the elimination of several opioids and their active metabolites, requiring careful dose selection during opioid rotation. Morphine produces morphine-6-glucuronide, an active metabolite that accumulates in renal failure and can cause prolonged sedation and respiratory depression. Codeine and tramadol metabolites also accumulate in renal failure. Hydromorphone produces hydromorphone-3-glucuronide, which can cause neuroexcitatory effects. Fentanyl and methadone are considered safer options in renal impairment because they are primarily hepatically metabolized with no clinically significant active metabolites that accumulate. When rotating opioids in renal patients, larger dose reductions of 50 percent or more may be appropriate, and extended dosing intervals should be considered. Frequent reassessment of pain control and adverse effects is essential.
What is the recommended approach for documenting opioid conversions?
Thorough documentation of opioid conversions is essential for patient safety and medicolegal protection and should include several key elements. The documentation should specify the source opioid with its exact dose, frequency, and route, the total daily morphine milligram equivalent dose calculated, the conversion ratio used with its source reference, the dose reduction percentage applied and the clinical rationale for the chosen percentage, the final target opioid dose with frequency and route, the breakthrough pain plan including medication, dose, and maximum frequency, and the monitoring plan including follow-up timeline. Additionally, the clinical rationale for the opioid rotation should be clearly stated, whether for inadequate pain control, intolerable side effects, or route change. All conversations with the patient about risks and benefits should be documented along with the informed consent process.
What are the most common unit conversion mistakes?
Common errors include confusing fluid ounces with weight ounces, mixing up miles and nautical miles, forgetting that UK and US gallons differ (UK is 20% larger), using the wrong temperature formula, and not accounting for the difference between troy and avoirdupois ounces.