Case Presentation: The patient was a 60-year-old woman with rheumatoid arthritis treated with methotrexate and abatacept infusions, CAD, HFpEF, COPD, and DM2. She initially received an outpatient prescription for Bactrim for a presumed UTI. Shortly afterward, she developed worsening fatigue, oral pain, confusion, and poor intake. On presentation she was found to have profound pancytopenia with an ANC of 0, severe mucositis. Her encephalopathy rapidly progressed, and developed concerns of inability to protect her airway and was intubated. During intubation, the team discovered extensive mucositis with blood and debris obstructing the right mainstem bronchus, requiring bronchoscopy and lavage. She also entered septic shock. Broad-spectrum antimicrobial coverage was initiated.Her hospital course was further complicated by the development of Candida tropicalis fungemia, prompting initiation of micafungin. She exhibited diffuse mucocutaneous candidiasis, consistent with severe immunosuppression. Despite clearance of fungemia and hemodynamic improvement, her neurological status did not recover. Continuous EEG monitoring revealed burst suppression without seizures, and MRI showed a small acute right frontal infarct.Her pancytopenia remained profound, despite transfusion. A bone marrow biopsy revealed a markedly hypocellular marrow (~15% cellularity) with trilineage hematopoiesis, compatible with multifactorial marrow failure driven by the combined toxicity of methotrexate and Bactrim, compounded by infection and severe folate deficiency. Additional complications included a right flank hematoma from recent falls, electrolyte disturbances, AKI that improved with fluids, and intermittent cardiac arrhythmias including SVT. She remained ventilator-dependent and was unable to participate in spontaneous breathing trials due to her persistent encephalopathy.Despite aggressive supportive care—including antimicrobial therapy, transfusions, nutritional support, and correction of metabolic abnormalities—her neurological function and bone marrow recovery remained severely impaired. Her prognosis grew increasingly grim as she continued to exhibit refractory pancytopenia, ventilator dependence, and failure to regain consciousness. She unfortunately was unable to recover.
Discussion: Bactrim is a common antibiotic used to treat a wide variety of infections. While one typically associates it with risks such as hyperkalemia or G6PD deficiency, it has a darker side. Because Bactrim inhibits folate pathways—similar to methotrexate—we must remember that combining these medications can profoundly impair bone marrow function. This case highlights an unfortunate and severe outcome of that interaction.
Conclusions: A key learning point from this case is the importance of careful antibiotic selection and thorough medication reconciliation. This outcome might have been avoided had the drug interaction between methotrexate and Bactrim been recognized before prescribing. Another important teaching point is determining when treatment of a UTI is truly indicated. Antibiotics are not always warranted—particularly in nonpregnant patients without clear urinary symptoms or high-risk conditions. Overall, this case underscores the necessity of individualized prescribing and meticulous medication review to prevent avoidable and life-threatening complications.