What is Equianalgesic Dosing?
Equianalgesic dosing refers to the dose of one opioid that provides the same level of pain relief (analgesia) as a given dose of another opioid. This concept is fundamental to opioid rotation — the practice of switching from one opioid to another when a patient experiences inadequate pain control, intolerable side effects, or when a specific formulation is needed.
The equianalgesic table uses oral morphine as the reference standard. All other opioids are compared against a standard dose of 30 mg oral morphine. These ratios, while based on clinical studies, should be considered approximate guidelines rather than precise conversion factors, as individual patient responses vary significantly.
Opioid rotation is one of the most common interventions in pain management, but it carries inherent risk. Incomplete cross-tolerance between opioids means that switching agents can sometimes result in unexpected potency differences, leading to either inadequate pain relief or dangerous over-sedation.
Conversion Formula
The multiplication by 0.75 represents a 25% dose reduction to account for incomplete cross-tolerance. This safety reduction is a standard recommendation in clinical guidelines from the American Pain Society and other organizations.
The conversion process works in two steps:
- Step 1: Convert the current opioid dose to oral morphine equivalents (MME) by dividing by the equianalgesic factor and multiplying by 30
- Step 2: Convert from oral morphine equivalents to the target opioid dose, then apply the 25% reduction
Equianalgesic Dose Table
The following table shows the dose of each opioid that is approximately equivalent to 30 mg of oral morphine:
| Opioid | Route | Equianalgesic Dose | Equiv to Morphine PO 30 mg |
|---|---|---|---|
| Morphine | PO (oral) | 30 mg | 1:1 (reference) |
| Morphine | IV / IM / SC | 10 mg | 3:1 PO:IV ratio |
| Codeine | PO | 200 mg | ~0.15x morphine |
| Hydrocodone | PO | 30 mg | 1:1 with morphine PO |
| Hydromorphone | PO | 6 mg | ~5x morphine PO |
| Hydromorphone | IV | 1.5 mg | ~20x morphine PO |
| Oxycodone | PO | 20 mg | ~1.5x morphine PO |
| Oxymorphone | PO | 10 mg | ~3x morphine PO |
| Fentanyl | Transdermal | 12.5 mcg/hr | Patch to MME varies |
| Methadone | PO | Variable* | Non-linear ratio |
| Tramadol | PO | 300 mg | ~0.1x morphine PO |
*Methadone conversions are highly variable and dose-dependent. This calculator uses simplified ratios. Consult a pain management specialist for methadone conversions at higher doses.
Opioid Potency Diagram
Cross-Tolerance Explained
Cross-tolerance occurs when tolerance to one opioid partially extends to another opioid. However, cross-tolerance between opioids is incomplete — typically only about 50–75% of the tolerance transfers between agents. This means that when rotating to a new opioid, the patient may be more sensitive to the new drug than the equianalgesic table would suggest.
To account for incomplete cross-tolerance, clinical guidelines recommend reducing the calculated equianalgesic dose by 25–50%:
- 25% reduction (multiply by 0.75): Standard recommendation for most opioid rotations when pain is adequately controlled
- 50% reduction (multiply by 0.50): Recommended for elderly patients, those with renal/hepatic impairment, or when switching to/from methadone
- No reduction: May be considered when rotating due to inadequate pain control (dose can be titrated from the full equianalgesic dose)
Special Considerations: Methadone
Methadone is unique among opioids due to its complex pharmacology. It has a long and variable half-life (8–59 hours), acts on NMDA receptors in addition to mu-opioid receptors, and its equianalgesic ratio changes with dose. At higher morphine equivalent doses, methadone becomes disproportionately more potent:
| Total Daily Morphine Equivalent | Morphine:Methadone Ratio |
|---|---|
| < 90 mg | 4:1 |
| 90 – 300 mg | 8:1 |
| 301 – 600 mg | 12:1 |
| 601 – 800 mg | 15:1 |
| > 800 mg | 20:1 |
Due to this complexity, methadone conversions should always be managed by an experienced clinician, ideally in consultation with a clinical pharmacist or pain specialist.
Worked Example
A patient currently takes Morphine PO 30 mg/day and needs to be switched to Oxycodone PO:
Step 2: Oxycodone equiv = (30 ÷ 30) × 20 = 20 mg
Step 3: With 25% reduction = 20 × 0.75 = 15 mg Oxycodone PO/day
The patient should be started on approximately 15 mg of oral Oxycodone per day (e.g., 5 mg every 8 hours or 7.5 mg every 12 hours), then titrated based on pain control and side effects.
Frequently Asked Questions
Why apply a 25% dose reduction when switching opioids?
The 25% reduction accounts for incomplete cross-tolerance between opioids. Each opioid has a unique receptor binding profile, so tolerance developed to one agent does not fully transfer to another. Starting at a lower dose and titrating up is the safest approach to avoid overdose while ensuring adequate pain control.
Is the equianalgesic table exact?
No. Equianalgesic tables are based on population averages from clinical studies and serve as starting guidelines. Individual responses vary based on genetics (CYP enzyme polymorphisms), age, organ function, prior opioid exposure, and concurrent medications. The tables should be used as a starting point, not an absolute conversion.
Can I convert directly between two non-morphine opioids?
Yes. The conversion goes through the morphine equivalent as an intermediate step. First convert the current opioid to oral morphine equivalents, then convert from morphine equivalents to the target opioid. This calculator performs both steps automatically.
When should I NOT use an equianalgesic table?
Equianalgesic tables may be unreliable for: (1) patients on very high doses of opioids, (2) methadone conversions (which require specialized protocols), (3) patients with significant renal or hepatic impairment (which affects drug metabolism and clearance), and (4) patients on concurrent CYP enzyme inhibitors or inducers.
What is Morphine Milligram Equivalent (MME)?
MME is a standardized value that expresses the dose of any opioid as the equivalent dose of oral morphine. It is used by healthcare providers, insurance companies, and regulatory agencies (such as the CDC) to quantify total opioid exposure. The CDC guidelines flag prescriptions exceeding 90 MME/day as requiring careful justification and monitoring.