Relative Strengths of Opioids
Oral morphine is the reference standard against which all other opioids are compared, with relative potency ratios allowing for equianalgesic conversions between different opioids. 1
Strong Opioids (WHO Level III)
| Substance | Route | Relative Effectiveness Compared to Oral Morphine | Starting Dose Without Pretreatment |
|---|---|---|---|
| Morphine sulfate | Oral | 1 | 20-40 mg |
| Morphine | Parenteral (IV/IM) | 3 | 5-10 mg |
| Oxycodone | Oral | 1.5-2 | 20 mg |
| Hydromorphone | Oral | 7.5 | 8 mg |
| Fentanyl | Transdermal | 4 | 12-25 μg/h |
| Buprenorphine | Oral | 75 | 0.4 mg |
| Buprenorphine | IV | 100 | 0.3-0.6 mg |
| Buprenorphine | Transdermal | 1.7 | 17.5-35 μg/h |
| Methadone | Oral | 4-12* | 10 mg |
| Nicomorphine | Oral | 1 | 5 mg |
| Nicomorphine | IV | 3 | 5 mg |
*Methadone conversion factor varies: 4 for daily morphine doses <90 mg, 8 for doses 90-300 mg, and 12 for >300 mg 1
Weak Opioids (WHO Level II)
| Substance | Relative Effectiveness Compared to Oral Morphine | Duration of Effectiveness (h) | Maximum Daily Dose | Starting Dose Without Pretreatment |
|---|---|---|---|---|
| Dihydrocodeine | 0.17 | 12 | 240 mg | 60-120 mg |
| Codeine | Not specified | 4-6 | 360 mg | 15-60 mg |
| Tramadol | 0.1-0.2 | 2-4 (immediate) / 12 (modified) | 400 mg | 50-100 mg |
Conversion Considerations
When converting between opioids, several important factors must be considered:
Genetic Variability: The potency of codeine and tramadol is strongly influenced by CYP2D6 genotype, which varies widely between individuals 2. This explains reports of overdosing or underdosing after administration of standard doses.
Cross-Tolerance: When switching between opioids, incomplete cross-tolerance often requires dose reduction of 25-50% from the calculated equianalgesic dose.
Methadone Complexity: Methadone conversions are particularly complex, with the conversion ratio increasing as the daily morphine dose increases 1. Use extreme caution with methadone due to its long and variable half-life.
Fentanyl Considerations: When converting to transdermal fentanyl, use conservative conversion tables as provided in the FDA labeling 3. For delivery rates exceeding 100 mcg/hr, multiple patches may be used.
Equianalgesic Conversion Table for Common Opioids
| Opioid | Oral Dose (mg) | Parenteral Dose (mg) |
|---|---|---|
| Morphine | 60 (30)* | 10 |
| Hydromorphone | 7.5 | 1.5 |
| Methadone | 20 | 10 |
| Oxycodone | 30 | 15 |
| Codeine | 200 | 130 |
*The conversion ratio varies based on clinical context 3
Common Pitfalls in Opioid Conversions
Significant Variability: There is substantial variability in equianalgesic calculations among healthcare providers, particularly for fentanyl and methadone 4. Always double-check calculations.
Ceiling Effects: Partial agonists and agonist-antagonists (like buprenorphine) have ceiling effects and don't have the broad dosage range of full agonists like morphine 5.
Underestimation of "Weak" Opioids: So-called weak opioids can have the same dose-dependent adverse effects as morphine. There is no evidence that, at equivalent analgesic efficacy, weak opioids carry a lower risk of addiction than low-dose morphine 2.
Methadone Complexity: The conversion factor for methadone varies based on the prior morphine dose, making it particularly challenging to convert accurately 1.
Transdermal Systems: When using transdermal systems like fentanyl patches, remember that it takes approximately 12-24 hours to reach steady-state plasma concentrations 3.
When managing pain with opioids, always titrate doses carefully, provide breakthrough dosing (typically 10% of the total daily dose), and reassess frequently for both efficacy and side effects 1.