Pharmacology
Opioid Analgesics for OPRA: Mechanism, Safety and MCQs
Opioids are tested constantly on OPRA because a single mechanism — mu-opioid receptor activation — produces both the pain relief and every major adverse effect, which is exactly the kind of connected reasoning OPRA rewards. This guide covers the WHO ladder, adverse effect management, naloxone reversal, and the codeine/oxycodone specifics that come up repeatedly.
Why this topic matters
Opioids sit at the intersection of Pharmacology and Therapeutics, and OPRA tests them from both directions — the receptor mechanism itself, and the practical safety/counselling consequences that mechanism creates. Understanding the single-receptor link between relief and risk makes most opioid questions reason out rather than require separate memorisation.
Learning objectives
- Explain why mu-opioid receptor activation produces both analgesia and the major adverse effects from a single mechanism
- Apply the WHO analgesic ladder to select an appropriate starting point for a given pain severity
- Identify which opioid adverse effect requires routine prophylaxis rather than reactive management
- State the naloxone dosing and duration caveats relevant to reversing a long-acting opioid
- Recognise the codeine CYP2D6 ultra-rapid metaboliser risk and the oxycodone-to-morphine potency ratio
Core concepts
One receptor, both the relief and the risk
Opioids can act at kappa and delta receptors too, but the mu-opioid receptor is by far the most clinically important — and the one OPRA focuses on. That single receptor produces both the wanted effect (analgesia) and the major unwanted effects: respiratory depression, sedation, constipation, miosis, and euphoria/dependence potential. This is why naloxone — a mu-receptor antagonist — reverses opioid effects mediated through that receptor, particularly respiratory and CNS depression, rather than selectively undoing only the dangerous ones. The degree of reversal in a given patient still depends on the specific opioid involved, the dose, and any ongoing opioid absorption.
The WHO analgesic ladder
The ladder matches opioid strength (and whether an opioid is used at all) to pain severity, using the least receptor activation needed for adequate control:
- Step 1 (mild pain) — paracetamol and/or an NSAID.
- Step 2 (mild-to-moderate pain) — a weak opioid (e.g. codeine, tramadol), typically combined with Step 1 agents.
- Step 3 (moderate-to-severe pain) — a strong opioid (e.g. morphine, oxycodone, fentanyl), typically combined with Step 1 agents.
- Adjuvants — antidepressants, anticonvulsants, corticosteroids, or topical agents — can be added at any step, and are the primary approach (not opioids) for neuropathic pain specifically.
Two drugs worth knowing individually
Codeine is a prodrug — CYP2D6 converts it to morphine. CYP2D6 ultra-rapid metabolisers (a genetic variant) convert an unusually large fraction of a codeine dose to morphine, risking opioid toxicity even at a standard dose. Oxycodone is approximately 1.5 times more potent than oral morphine on a milligram-for-milligram basis (20 mg oral oxycodone ≈ 30 mg oral morphine) — a specific ratio worth knowing, though like any opioid conversion it's an approximation, and individual patient factors (renal/hepatic function, prior opioid exposure) mean it should be applied cautiously in practice, not treated as an exact substitution.
Tramadol is more than a weak opioid
Tramadol is often grouped with weak opioids on the WHO ladder, but its mechanism is more complex than that grouping suggests: it's a weak mu-opioid agonist, and it also inhibits serotonin and noradrenaline reuptake. Those extra mechanisms — not the opioid effect alone — are what explain tramadol's specific additional risks: serotonin syndrome (particularly in combination with SSRIs or other serotonergic drugs) and a lowered seizure threshold.
Clinical application
The one adverse effect that needs prophylaxis, not reaction
Opioid-induced constipation does not resolve with tolerance the way sedation and nausea typically do — it persists for as long as the opioid is used. Stimulant laxatives (e.g. senna, bisacodyl) are first-line and co-prescribed routinely from the start of opioid therapy, rather than waiting to see if constipation develops, often with docusate; an osmotic laxative can be added if a stimulant laxative alone isn't enough. Lactulose alone is usually not the preferred choice for this specific indication.
Naloxone reversal — the timing trap
400 micrograms IV/IM, repeated every 2–3 minutes as needed, is a reasonable teaching dose for naloxone — but real practice is more nuanced. In hospital, IV dosing is often titrated in smaller increments specifically to restore adequate ventilation without precipitating severe, distressing withdrawal in an opioid-dependent patient, and community take-home naloxone products come in different formulations and doses. The key safety detail regardless of the exact dose used: naloxone's duration of action (roughly 60–90 minutes) is shorter than most opioids' duration of effect, so a patient can re-sedate and re-develop respiratory depression once the naloxone wears off — repeat dosing or an infusion may be needed, particularly for long-acting opioids.
Common mistakes
- Treating opioid-induced constipation reactively (waiting for symptoms) instead of prescribing a stimulant laxative prophylactically from the start.
- Assuming a single dose of naloxone permanently reverses an overdose, when its shorter duration relative to most opioids means re-sedation is a real risk.
- Treating a fixed oral-to-parenteral morphine conversion ratio as settled — Australian sources vary (commonly 2:1 to 3:1), so this should be treated as needing local protocol confirmation rather than a single memorised number.
- Reaching for an opioid as first-line for neuropathic pain, when adjuvants (antidepressants, anticonvulsants) are the primary approach for that pain type.
Exam tips
- • If a scenario describes both good pain relief and a concerning adverse effect (e.g. sedation) from the same opioid, that's testing the shared mu-receptor mechanism — the two aren't separable pharmacologically.
- • The oxycodone-to-morphine 1.5:1 ratio is one of the few opioid conversion figures worth memorising precisely, since — unlike the oral-to-parenteral morphine ratio — it isn't a genuinely disputed figure in the literature.
- • A naloxone scenario asking what happens "some time after" reversal is almost always testing the duration-mismatch/re-sedation risk, not a straightforward "the overdose is resolved" answer.
Memory tricks
- • "Same lock, same key" — one mu-receptor, so one antagonist (naloxone) undoes both the relief and the risk together.
Clinical pearls
- 💡 Neuropathic pain is the WHO ladder's deliberate exception: because it arises from nerve hyperexcitability rather than nociceptor activation, it responds better to antidepressants/anticonvulsants than to escalating opioid doses — a scenario testing neuropathic pain management is usually checking whether you reach for an opioid by habit or select the mechanistically appropriate option.
Tables
WHO analgesic ladder
| Step | Pain severity | Typical choice |
|---|---|---|
| 1 | Mild | Paracetamol ± NSAID |
| 2 | Mild-to-moderate | Weak opioid (codeine, tramadol) ± Step 1 |
| 3 | Moderate-to-severe | Strong opioid (morphine, oxycodone, fentanyl) ± Step 1 |
Opioid adverse effects and management approach
| Effect | Management |
|---|---|
| Constipation | Prophylactic stimulant laxative from the start — does not resolve with tolerance |
| Nausea | Typically improves after 1–2 weeks; antiemetic if needed in the interim |
| Sedation | Usually improves with tolerance; dose reduction or opioid rotation if persistent |
| Respiratory depression | Naloxone; monitor for re-sedation given naloxone's shorter duration of action |
Practice MCQs (100% original)
1. Why does naloxone reverse both the analgesic effect and the respiratory depression of an opioid overdose simultaneously?
2. A patient is started on regular oxycodone for moderate-to-severe cancer pain. What should be co-prescribed from the outset, rather than added reactively if symptoms develop?
3. A patient with ultra-rapid CYP2D6 metaboliser status is prescribed a standard dose of codeine. What is the MOST likely consequence?
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Start freeFrequently asked questions
Is there a single correct oral-to-parenteral morphine conversion ratio?
Not a universally agreed one — Australian sources and local protocols vary, commonly between 2:1 and 3:1. Rather than memorising a single figure as definitively correct, treat this as a value to confirm against the specific guideline or local protocol in use, and expect any exam scenario testing this to either specify which ratio to use or focus on a more settled figure such as the oxycodone-to-morphine 1.5:1 potency ratio.
Why is opioid-induced constipation treated differently from other opioid adverse effects?
Most opioid adverse effects (sedation, nausea) improve as tolerance develops with continued use. Constipation is the exception — it does not resolve with tolerance for as long as the opioid is used, which is why a stimulant laxative is prescribed prophylactically from the start rather than reactively.
Official references
- Therapeutic Guidelines Australia — Analgesic ↗ — WHO ladder application and opioid selection in Australian practice
- Australian Medicines Handbook — Opioid analgesics ↗ — Drug-specific dosing, adverse effects and monitoring