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Background and Importance Adult-onset Still’s disease (AOSD) is a rare autoinflammatory disorder that may present with life-threatening complications such as interstitial lung disease and macrophage activation syndrome (MAS), which occurs in only 8-14% of patients. Management is challenging due to disease relapses, multimorbidity, and drug-related toxicities. Pneumocystis jirovecii pneumonia (PJP) prophylaxis is required in immunosuppressed patients, although contraindications to first-line agents may force the use of alternatives with potential toxicity.Aim and Objectives We report the case of a 72-year-old woman with AOSD (involving skin, lungs and eyes) on long-term anakinra treatment who developed severe dapsone-induced metahemoglobinemia. In May 2025, glucocorticoid therapy was initiated for pulmonary disease progression. Due to history of cotrimoxazole-induced Stevens-Johnson syndrome, cotrimoxazole was contraindicated for PJP prophylaxis and dapsone was prescribed instead after confirming normal glucose-6-dehydrogenase (G6PD) activity. After 5-6 weeks of treatment the patient presented to the emergency department with fever, respiratory failure, cyanosis, anaemia and livedo reticularis.Material and Methods Laboratory tests on admission showed leukocytosis, thrombocytopenia, and elevated inflammatory markers. Methemoglobinemia reached 22%. She was diagnosed of dapsone-induced methemoglobinemia. Therefore, dapsone was discontinued and metahemoglobinemia treated with IV methylene blue and ascorbic acid, showing rapid improvement. During hospitalisation, she developed MAS with hyperferritinemia (117,200 ng/mL), hypertriglyceridemia (393 mg/dL), hypofibrinogenemia (1,64 g/L), and cytopenias (Hb 88 g/L, platelets 10×10^9/L) and plasma sCD25 levels of 4165 UI/mL. For MAS treatment, she required intensive care treatment with high-dose methylprednisolone (500 mg/day for 3 days, followed by oral corticosteroids in a tapering regimen at 1 mg/Kg/day), tocilizumab (8 mg/kg), and anakinra (200 mg/day). The clinical pharmacist followed the patient throughout admission, supporting immunosuppressive adjustments, and contributing to the therapeutic consensus that ensured an individualised treatment.Results The patient achieved clinical remission of MAS after intensive immunosuppressive therapy. At discharge, anakinra was switched to monthly tocilizumab due to persistent thrombocytopenia. For PJP prophylaxis, the patient is currently treated with monthly inhaled pentamidine. Treatment was tolerated without further adverse events.Conclusion and Relevance This case highlights the complexity of AOSD treatment in fragile elderly patients, where overlapping diseases, prophylactic constraints and drug toxicities complicate management. Pharmacist involvement is essential to identify safer prophylactic alternatives, optimise immunomodulation, and support multidisciplinary decision making.Conflict of Interest No conflict of interest