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Trimethoprim Folate Deficiency — SCE Infectious Diseases MCQ

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EasyAntimicrobial StewardshipTrimethoprim Folate DeficiencySCE Infectious Diseases

A 55-year-old woman with a history of recurrent UTIs is prescribed long-term low-dose Trimethoprim 100 mg nocte as prophylaxis. After 3 months, she develops megaloblastic anaemia. What is the mechanism?

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Correct answer: B — Trimethoprim is a folate antagonist — long-term use can cause folate deficiency leading to megaloblastic anaemia

Trimethoprim inhibits dihydrofolate reductase (DHFR), blocking folate metabolism. While this is the basis of its antibacterial activity (bacterial DHFR is much more susceptible than human DHFR), prolonged use can cause clinically significant folate depletion in humans, leading to megaloblastic anaemia, pancytopenia, and neutropenia. Risk factors include pre-existing folate deficiency, malnutrition, and concurrent folate antagonists (Methotrexate). FBC monitoring should be performed during long-term Trimethoprim use. Folic acid supplementation may be considered.

Reference: BNF 2024 – Trimethoprim; NICE NG112 2018 – Recurrent UTI