[Fatal pancytopenia and methotrexate-trimethoprim-sulfamethoxazole interaction].

Low dose methotrexate [MTX] is now frequently used for various inflammatory diseases. This is a case study of a fatal outcome in a patient with rheumatoid arthritis [RA] treated for a short period with low dose MTX. The patient developed severe pancytopenia followed by bacterial and monilial sepsis upon the co-administration of trimethoprim-sulphamethoxazole [TMP-SMX] for an intercurrent infection. The differential diagnosis of pancytopenia and the mechanisms underlying the increase in plasma free MTX by MTX-SMX in the patient are discussed. It should be noted that this fatal case highlights the risk of severe drug interactions in patients with multiple risk factors treated with low dose MTX for a short period of time.