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Allopurinol and azathioprine — GPhC CRA MCQ

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HardMusculoskeletal TherapeuticsAllopurinol and azathioprineGPhC CRA

A 63-year-old man with gout is prescribed allopurinol 100 mg daily after his third flare this year. He takes azathioprine for ulcerative colitis and has eGFR 55 mL/min/1.73 m2. What is the most important interaction to flag?

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Correct answer: A — Allopurinol can increase azathioprine toxicity through xanthine oxidase inhibition

Allopurinol inhibits xanthine oxidase, reducing metabolism of azathioprine and increasing risk of severe myelosuppression. Co-prescribing requires specialist dose adjustment and close monitoring or an alternative approach. Renal impairment adds further need for careful dosing and monitoring.

Reference: BNF; SPS; NICE CKS