Pharmacology
High-Yield Drug Interactions for OPRA
A small set of interaction patterns account for a disproportionate share of OPRA interaction questions — CYP450-mediated warfarin interactions, lithium toxicity precipitants, and serotonin syndrome combinations chief among them. This guide covers the mechanisms and the specific high-risk pairs worth knowing cold.
Why this topic matters
Drug interactions cause significant preventable harm in real practice, and OPRA tests them not as trivia but as a proxy for whether a candidate can reason about mechanism — predicting an interaction you haven't specifically memorised is a skill the exam rewards directly.
Learning objectives
- Classify a drug interaction as pharmacokinetic or pharmacodynamic
- Apply CYP450 inhibitor/inducer knowledge to predict the direction of a warfarin interaction
- Identify the drug classes most likely to precipitate lithium toxicity and explain the shared mechanism
- Recognise the serotonin syndrome triad and its common precipitating combinations
Core concepts
Pharmacokinetic vs pharmacodynamic interactions
A pharmacokinetic interaction changes how much of a drug reaches its site of action — through altered absorption, metabolism (commonly via CYP450 enzymes), or elimination. A pharmacodynamic interaction changes the effect of a drug at its site of action without necessarily changing its concentration — two drugs with additive or opposing effects on the same physiological system. Recognising which category an interaction falls into is often the fastest route to reasoning out an unfamiliar combination.
CYP450 inhibitors and inducers — the warfarin pattern
Warfarin is metabolised substantially via CYP2C9, making it one of the most reliably tested CYP450 interaction examples. CYP2C9 inhibitors (e.g. fluconazole, amiodarone, metronidazole, fluoxetine) reduce warfarin metabolism, raising INR and bleeding risk. CYP2C9 inducers (e.g. rifampicin, carbamazepine, St John's Wort) increase warfarin metabolism, lowering INR and reducing anticoagulant effect. NSAIDs added to warfarin raise bleeding risk through a largely separate, additive mechanism (antiplatelet and GI-mucosal effects) rather than by significantly changing the INR itself.
Lithium toxicity — a renal-handling pattern, not a CYP450 one
Lithium is renally eliminated and not metabolised by CYP450 enzymes, so its major interactions work through a different mechanism: anything that reduces renal sodium/lithium clearance raises lithium levels. NSAIDs (via reduced renal prostaglandin synthesis, lowering GFR), thiazide diuretics (via compensatory proximal tubular reabsorption that carries lithium with it), and ACE inhibitors/ARBs (via reduced GFR) are the classic precipitants — as are dehydration and a low-salt diet, which work through the same renal-handling logic.
Clinical application
Recognising serotonin syndrome
Serotonin syndrome presents as a triad: hyperthermia, neuromuscular abnormalities (clonus, hyperreflexia, tremor), and altered mental status. Common precipitating combinations include an SSRI with tramadol, an SSRI with linezolid, an SSRI with a triptan, or an SSRI with high-dose fentanyl. Management centres on stopping the serotonergic agents; cyproheptadine is used as a specific antidote in more severe cases.
QT prolongation — an additive-risk pattern
QT-prolongation risk with combinations of QT-prolonging drugs is additive rather than all-or-nothing — antipsychotics (e.g. haloperidol, quetiapine), azithromycin, methadone, azole antifungals, and Class Ia/III antiarrhythmics are all recognised contributors. A scenario stacking two or more of these is testing whether the cumulative risk is recognised, not whether any single one is individually dangerous.
Common mistakes
- Assuming every clinically important interaction is CYP450-mediated — lithium's key interactions work through renal handling, not hepatic metabolism.
- Treating an NSAID-warfarin interaction as an INR-level (pharmacokinetic) problem, when the bleeding-risk increase is predominantly an additive, largely pharmacodynamic effect.
- Missing that CYP450 inducers and inhibitors move a warfarin-affected INR in opposite directions — a frequently tested, easily reversed detail under time pressure.
- Overlooking that QT-prolongation risk accumulates across multiple contributing drugs rather than depending on any single agent alone.
Exam tips
- • When a stem lists a new drug being added to warfarin, identify whether it's a CYP2C9 inhibitor or inducer first — that single classification predicts the direction of the INR change.
- • A scenario describing hyperthermia plus clonus/hyperreflexia plus confusion, in a patient on multiple serotonergic drugs, is a serotonin syndrome pattern-recognition question, not a differential-diagnosis one.
Memory tricks
- • "Warfarin = liver (CYP2C9), lithium = kidney" — a short line to keep the two most-tested interaction mechanisms from being confused with each other.
Clinical pearls
- 💡 NSAIDs appear in both the warfarin and lithium high-yield interaction lists, but through entirely different mechanisms in each case (additive bleeding risk vs reduced renal clearance) — a useful example of why classifying the mechanism matters more than memorising drug-pair lists alone.
Tables
Warfarin — CYP2C9 interaction direction
| Interacting drug type | Examples | Effect on INR |
|---|---|---|
| CYP2C9 inhibitors | Fluconazole, amiodarone, metronidazole, fluoxetine | INR increases (bleeding risk) |
| CYP2C9 inducers | Rifampicin, carbamazepine, St John's Wort | INR decreases (reduced anticoagulation) |
| NSAIDs | Ibuprofen, diclofenac, naproxen | Bleeding risk increases (largely independent of INR) |
Lithium toxicity precipitants and shared mechanism
| Precipitant | Mechanism |
|---|---|
| NSAIDs | Reduced renal prostaglandins → reduced GFR |
| Thiazide diuretics | Compensatory proximal reabsorption carries lithium with it |
| ACE inhibitors / ARBs | Reduced GFR |
| Dehydration / low-salt diet | Same renal-handling logic — reduced lithium clearance |
Practice MCQs (100% original)
1. A patient stable on warfarin is started on fluconazole for a fungal infection. What is the MOST likely effect on their INR?
2. A patient on long-term lithium therapy is prescribed regular ibuprofen for osteoarthritis. What is the most likely consequence?
3. A patient on an SSRI is started on tramadol and develops hyperthermia, clonus and confusion within hours. What is the most likely diagnosis?
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Start freeFrequently asked questions
How can I predict an unfamiliar drug interaction on the exam if I haven't memorised that specific pair?
Classify the interaction type first (pharmacokinetic vs pharmacodynamic), then identify the mechanism each drug is known for (e.g. CYP450 effect, renal clearance effect, serotonergic activity, QT effect). Most OPRA interaction questions are testing whether you can apply a known mechanism to a new pairing, not whether you've memorised an exhaustive list.
Is cyproheptadine used for all serotonin syndrome cases?
Cyproheptadine is used as a specific antidote in more significant cases; mild cases are often managed by simply stopping the serotonergic agents and providing supportive care. Severity guides the management approach.
Official references
- Australian Medicines Handbook ↗ — Drug-specific interaction detail
- Therapeutic Guidelines Australia ↗ — Clinical management of significant drug interactions