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Trimethoprim hyperkalaemia — GPhC CRA MCQ

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HardDrug InteractionsTrimethoprim hyperkalaemiaGPhC CRA

A 74-year-old man taking ramipril 10 mg daily and spironolactone 25 mg daily is prescribed trimethoprim for UTI. His eGFR is 46 mL/min/1.73 m² and potassium last week was 5.1 mmol/L. What is the most important interaction to flag?

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Correct answer: D — Trimethoprim can increase hyperkalaemia risk with ACE inhibitor and spironolactone

Trimethoprim can act like a potassium-sparing diuretic and increase potassium. The risk is higher with ACE inhibitors, spironolactone, renal impairment and a baseline potassium of 5.1 mmol/L. The prescriber should review antibiotic choice and monitoring before supply. This is a potentially serious interaction in an older patient.

Reference: BNF; NICE CKS