Methotrexate and co-trimoxazole — GPhC CRA MCQ
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Correct answer: C — Query co-trimoxazole because combined antifolate toxicity can suppress marrow
Trimethoprim and co-trimoxazole can cause severe, sometimes fatal marrow suppression with methotrexate through additive antifolate effects and altered handling. A recent normal FBC and routine folic acid do not remove the risk; an alternative antibiotic is needed.
Reference: BNF: methotrexate: https://bnf.nice.org.uk/drugs/methotrexate/