Opioid Equivalence Calculator
Morphine Milligram Equivalent (MME)
Your Conversion Summary
Equianalgesic Doses
These doses should produce equivalent analgesia to your input dose. Reduce by 25-50% if switching opioids (tolerance may not cross fully). Adjust based on clinical response.
Breakthrough Dosing
How Each Opioid Works (NZ Context)
Dose Switching & Risk Assessment
First-line for acute pain: Oral immediate-release morphine or oxycodone (good bioavailability, short onset).
Chronic pain (cancer/non-cancer): Modified-release morphine or oxycodone once stable on IR. Transdermal fentanyl/buprenorphine for patients with swallowing difficulty or poor compliance. Methadone and buprenorphine require specialist input.
Weak opioids (codeine, tramadol, dihydrocodeine): Limited efficacy; codeine conversion is non-linear and genetically variable (CYP2D6). Reserve for mild-moderate pain unresponsive to non-opioid analgesia.
Fentanyl patches: Reserve for opioid-tolerant patients (≥60 mg morphine/day) due to risk of overdose in opioid-naive patients. Patches take 12-24 h to reach steady state; bridge with IR opioid.
Buprenorphine patches: Lower overdose risk than fentanyl (ceiling effect on respiratory depression). Useful in older adults and patients with respiratory compromise. Avoid mixing with strong opioids (competitive antagonism).
- Renal impairment: Reduce dose for morphine, codeine, tramadol (accumulation of active metabolites). Fentanyl/methadone less affected.
- Hepatic impairment: Use with caution; reduce dose. Methadone requires particular care.
- Age >70 years: Reduce by 25-50%; lower metabolism, higher comorbidity burden.
- Drug interactions: CYP3A4 inhibitors (ketoconazole, ritonavir) increase fentanyl/methadone levels. Avoid with CNS depressants (benzodiazepines, alcohol) due to respiratory depression risk.
- Opioid-naive patients: Start low (e.g., 5-10 mg morphine IR) and titrate. Calculator assumes tolerance.
- Obesity: Fentanyl dosing may not scale linearly; patch absorption can be unpredictable.
- Switching from weak to strong opioid: Cannot reliably calculate; reduce strong opioid by 50% and titrate.
Constipation (80% of patients): Routine laxation with stimulant (bisacodyl) + osmotic (polyethylene glycol). Consider naloxegol (opioid antagonist) for refractory cases.
Nausea: Common in first 1-2 weeks; usually resolves. First-line: metoclopramide or cyclizine. Avoid antihistamines (increase constipation).
Drowsiness/cognitive impairment: Tolerance develops over days. If persistent, reduce dose or switch opioid. Stimulants (methylphenidate) rarely indicated.
Respiratory depression: Risk with rapid titration, renal/hepatic impairment, or concurrent CNS depressants. Monitor respiratory rate; avoid in severe COPD (seek respiratory specialist input).
Opioid-induced hyperalgesia: Rare; consider dose reduction or opioid rotation.
Physical dependence: Predictable; develops with regular use. Managed by slow tapering. Not addiction.
Tolerance: Need for dose increase over time to maintain effect. Normal with chronic opioid use; not a sign of failure or addiction.
Addiction (opioid use disorder): Loss of control over use, continued use despite harm, preoccupation with obtaining drug. Requires assessment using DSM-5 criteria and input from addiction specialist (many GPs in NZ have acquired competency). Opioid agonist therapy (methadone, buprenorphine) or buprenorphine/naloxone may be indicated.
NZ guidance: HealthPathways and NZ MoH Opioid Framework recommend universal precautions (baseline risk assessment, regular review, urine drug screening if high-risk), not universal suspicion. Cannabis co-use is common; not a contraindication but increases respiratory depression risk.
Equianalgesic Dosing Table
Doses that produce equivalent analgesia to 10 mg morphine IV/SC (parenteral reference). Oral doses are ~2-3�- higher due to first-pass metabolism.
| Opioid | Route | Equianalgesic Dose | Notes |
|---|---|---|---|
| Morphine | Oral (IR) | 30 mg | Gold standard; reference opioid |
| IV/SC | 10 mg | Parenteral reference | |
| Oxycodone | Oral (IR) | 20 mg | ~1.5�- more potent than morphine orally |
| Hydromorphone | Oral | 7.5 mg | 4�- more potent; shorter duration |
| Fentanyl | Transdermal (mcg/h) | 2.4 mcg/h ≈ 60 mg morphine/day | 100 mcg/h patch ≈ 2500 mg morphine/day |
| SL tablet/spray | 100-200 mcg | For breakthrough pain in opioid-tolerant | |
| Buprenorphine | Transdermal (mcg/h) | 7 mcg/h ≈ 30 mg morphine/day | Partial agonist; lower overdose risk |
| Methadone | Oral | 2-4 mg | Highly variable; specialist dosing required. Long half-life (24-36 h); risk of accumulation. |
| Codeine | Oral | 200 mg | Non-linear. Prodrug (CYP2D6 conversion); poor analgesic at high doses. Avoid >240 mg/day. |
| Tramadol | Oral | 100 mg | Also SNRI; seizure risk at high doses. Avoid in renal impairment (metabolite accumulation). |
| Dihydrocodeine | Oral | ~30 mg | Similar profile to codeine; better efficacy but less evidence. |
References & Guidance Documents
- NZ Ministry of Health. Opioid Medication Framework. Wellington: MoH; 2022. https://www.health.govt.nz
- HealthPathways New Zealand. Chronic Pain Management & Opioid Dosing. https://www.healthpathways.org.nz [cited 2024]
- Mercadante S. Opioid rotation for cancer pain: rationale and clinical aspects. Cancer. 1999;86(9):1856-66.
- Portenoy RK, Lesage P. Management of cancer pain. Lancet. 1999;353(9165):1695-700.
- Coffman CW. Equianalgesic dosing of opioids: a clinical reference. J Pain Palliat Care Pharmacother. 2015;29(2):152-8.
- New Zealand Pharmacovigilance Centre. Opioid-related adverse events: monitoring and reporting. Dunedin: University of Otago; [cited 2024].
- Ritter JM, Flower RJ, Henderson G, et al. Rang and Dale's Pharmacology. 9th ed. Edinburgh: Elsevier; 2020.