In March 2022, the New Zealand Pharma-covigilance Centre reported a case of fatal pancytopenia in a patient who received both methotrexate (a cytotoxic immunosuppressant) and trimethoprim (an antibiotic) at the same time (1). This drug interaction, which can have fatal consequences, has been recognised for several decades (2,3). Patients who are taking methotrexate, even at a low weekly dose, are exposed to a risk of pancytopenia if they also take cotrimoxazole, i.e. a combination of trimethoprim and sulfamethoxazole, a sul-fonamide (2).
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