Conversion tables are approximate. When switching between opioids, cross-tolerance is incomplete: reduce the calculated equianalgesic dose by 25–50 per cent and titrate. At a high dose, in old age or in renal failure, reduce it further.
Starting doses in acute pain in an opioid-naive adult
| Agent | Route | Starting dose | Comment |
|---|---|---|---|
| Morphine | IV | 2.5–5 mg, repeated after 5–10 min as needed | Halve the dose in older people and in renal failure |
| Morphine | PO | 5–10 mg as needed | The active metabolite accumulates in renal failure |
| Oxycodone | IV | 2.5–5 mg | |
| Oxycodone | PO | 5–10 mg as needed | The first choice among the common opioids in renal failure |
| Ketobemidone | IV | 2.5–5 mg | Oral bioavailability about 35 per cent |
| Paracetamol | PO or IV | 1 g × 4 | The basic analgesic; always give it in parallel unless contraindicated |
| NSAID | PO | According to the preparation | Avoid in renal failure, heart failure, a history of peptic ulcer, and old age |
Titrate according to the effect and the respiratory rate, not to a schedule. Always prescribe a laxative and an antiemetic as required at the same time as the opioid.
Approximate equianalgesic conversion
| Opioid | Oral daily dose | Parenteral daily dose |
|---|---|---|
| Morphine | 30 mg | 10 mg |
| Oxycodone | 20 mg | 10 mg |
| Hydromorphone | 4 mg | 2 mg |
| Ketobemidone | 30 mg | 10 mg |
Fentanyl patch: 12 micrograms/hour corresponds approximately to 25–30 mg of oral morphine per day, and 25 micrograms/hour to approximately 40–50 mg of oral morphine per day. Patches have no place in acute, unstable pain: the onset takes 12 to 24 hours and the offset just as long.
Breakthrough dose
During ongoing opioid treatment: a breakthrough dose of about one sixth of the daily dose in the same formulation, as required every 4 hours. Recalculate the daily dose when more than three breakthrough doses per day are needed.
Dose adjustment
| Situation | Adjustment |
|---|---|
| eGFR < 30 mL/min | Avoid morphine (the active metabolite accumulates). Oxycodone at a reduced dose, at longer intervals |
| Age > 75 years | Start at half the adult dose and lengthen the interval |
| Liver failure | A reduced dose and a longer interval for all opioids |
| Obesity | Dose according to effect, not according to total body weight |
| Concomitant benzodiazepine | A markedly increased risk of respiratory depression. Avoid the combination, or reduce both |
Red flags and pitfalls
- A respiratory rate < 8/min, or an increasing level of sedation, are signs of overdose. Sedation precedes respiratory depression; monitor it.
- Naloxone is titrated to spontaneous breathing, not to wakefulness. It has a short half-life, and the patient must be monitored afterwards.
- Starting a modified-release preparation before the daily requirement is known.
- Duplication. Check that the patient is not already on an opioid patch, a modified-release tablet and an as-required opioid at the same time.
- Opioids in abdominal pain of unclear cause do not delay the diagnosis and should be given, but document the examination findings before the dose.
- A plan for stopping must be documented when the drug is started.
Sources
- WHO guidelines on the pharmacological management of pain.
- The summary of product characteristics for each opioid preparation.
