Fatal Flaw in the Folate Pathway: Methotrexate-Bactrim Interaction, Misleading Levels, and a Cautionary Case.

Methotrexate (MTX) is one of the most commonly prescribed medications for rheumatoid arthritis and other autoimmune disorders. While therapeutic drug monitoring (TDM) protocols are well-established for high-dose MTX (HD-MTX) in oncology, no such validated protocols exist for low-dose MTX (LD-MTX) therapy - the pharmacokinetics are highly variable, and serum concentrations correlate poorly with clinical toxicity. Among the most dangerous drug interactions is the combination of MTX with trimethoprim-sulfamethoxazole (TMP-SMX), which creates synergistic toxicity. This case details a rapid, fatal MTX toxicity event in a patient on stable LD therapy who received a short course of TMP-SMX for cellulitis. Severe pancytopenia and hemorrhagic oral mucositis developed despite a serum MTX concentration of 0.07 μmol/L - well below the commonly accepted toxic threshold. This case underscores that clinicians must prioritize prompt recognition of clinical symptoms over laboratory quantification when risk factors for drug interaction are present.