Search Assignment Sample For Ireland Students

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This MD3005 pharmacology sample shows one way to structure a 500-word answer to the MD3005 pharmacology written assignment on a pharmacologically challenging patient case. The case below is entirely fictional and anonymised, and every clinical figure in it is illustrative. For the thinking behind each section, read how to approach a pharmacologically challenging patient case. A related UCC brief is the paramedic pharmacology polypharmacy case question.

Planning table

Model answer extract

Case description (fictional)

This is a fictional case written for illustration. A 78-year-old woman with atrial fibrillation had a stable INR of about 2.5 on long-term warfarin. She was prescribed clarithromycin 500 mg twice daily for a lower respiratory tract infection. Five days later she developed epistaxis and widespread bruising, and her INR was 6.8 (illustrative figure).

Drug 1: warfarin

Warfarin inhibits regeneration of vitamin K1 epoxide, reducing synthesis of the functional clotting factors II, VII, IX and X and of proteins C and S (Strides Pharma UK Ltd, 2025). Because existing factors must first be cleared, the effect lags behind dosing.

Absorption is essentially complete, with bioavailability above 90%. About 99% is bound to plasma protein and the apparent volume of distribution is roughly 0.14 L/kg. Warfarin is a racemate: S-warfarin is cleared mainly by CYP2C9, and the less active R-warfarin by CYP1A2 and CYP3A4 (Strides Pharma UK Ltd, 2025; Juurlink, 2007). The effective half-life is about 40 hours, and metabolites are excreted mainly in urine.

The principal adverse effect is haemorrhage, including gastrointestinal and intracranial bleeding. The narrow therapeutic index is why INR monitoring is required (Strides Pharma UK Ltd, 2025).

Drug 2: clarithromycin

Clarithromycin is a macrolide that binds the 50S ribosomal subunit and inhibits bacterial protein synthesis (Brown & Burk UK Ltd, 2025). It is well absorbed orally, and first-pass metabolism forms an active 14-hydroxy metabolite. About 80% is protein bound, and kinetics are non-linear. At 500 mg twice daily roughly 36% is excreted unchanged in urine, with the remainder eliminated mainly in bile (Brown & Burk UK Ltd, 2025).

Common adverse effects are nausea, diarrhoea, abdominal pain and taste disturbance. QT prolongation is also recognised. Importantly, warfarin is listed among oral anticoagulants metabolised by the CYP3A enzymes that clarithromycin inhibits, and frequent INR monitoring is advised (Brown & Burk UK Ltd, 2025).

Why the combination was challenging

This is mainly a pharmacokinetic interaction. Inhibition of CYP3A4 slows clearance of R-warfarin. Since R-warfarin is the less active enantiomer, this alone seems too small to explain an INR of 6.8, so disruption of vitamin K-producing gut flora may contribute (Juurlink, 2007; Holbrook et al., 2005). The mechanism is therefore not fully settled.

Warfarin’s narrow therapeutic index and extensive protein binding mean that a modest fall in clearance can produce a disproportionate rise in anticoagulant effect. In an observational study of older warfarin users, antibiotic exposure within 15 days was associated with roughly double the odds of bleeding, and the adjusted odds ratio for macrolides was 1.86 (Baillargeon et al., 2012).

Pharmacological implications

The patient experienced exaggerated warfarin effect and clinically significant bleeding risk. A broad response would be to withhold warfarin, monitor INR closely and use vitamin K, which restores clotting factor synthesis, if indicated. Choosing an antibiotic with less CYP3A4 inhibition would avoid repeating the problem (Ritter et al., 2024).

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References

Baillargeon, J., Holmes, H.M., Lin, Y.L., Raji, M.A., Sharma, G. and Kuo, Y.F. (2012) ‘Concurrent use of warfarin and antibiotics and the risk of bleeding in older adults’, American Journal of Medicine, 125(2), pp. 183–189.

Brown & Burk UK Ltd (2025) Clarithromycin 500 mg film-coated tablets: summary of product characteristics. Available at: https://www.medicines.org.uk/emc/product/10641/smpc (Accessed: 5 November 2026).

Holbrook, A.M., Pereira, J.A., Labiris, R. et al. (2005) ‘Systematic overview of warfarin and its drug and food interactions’, Archives of Internal Medicine, 165(10), pp. 1095–1106.

Juurlink, D.N. (2007) ‘Drug interactions with warfarin: what clinicians need to know’, Canadian Medical Association Journal, 177(4), pp. 369–371.

Ritter, J.M., Flower, R.J., Henderson, G., Loke, Y.K., MacEwan, D., Robinson, E. and Fullerton, J. (2024) Rang and Dale’s pharmacology. 10th edn. Elsevier.

Strides Pharma UK Ltd (2025) Warfarin 5 mg tablets: summary of product characteristics. Available at: https://www.medicines.org.uk/emc/product/14333/smpc (Accessed: 5 November 2026).