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Dapsone may be used for Pneumocystis jirovecii pneumonia (PJP) prophylaxis in hematology patients receiving immunosuppressive therapy or after hematopoietic stem cell transplant (HSCT) in the setting of trimethoprim-sulfamethoxazole (TMP-SMX) adverse drug reaction (ADR) history. Dapsone-induced hematological toxicities such as oxidative hemolysis may limit use in these patients and modern assessments of dapsone allergy cross-reactivity in non-HIV patients with a sulfonamide allergy are largely absent. The aim of this single-centre, retrospective study was to describe dapsone usage in hematology patients requiring PJP prophylaxis, including HSCT recipients, over a 12-month period in terms of indications, incidence of dapsone-attributed oxidative hemolysis, and immune cross-reactivity in those previously labeled with a sulfonamide allergy, as well as describing potential opportunities for first-line TMP-SMX PJP prophylaxis reintroduction. Of 24 patients meeting the study inclusion criteria, 12 (50%) were receiving dapsone PJP prophylaxis post-HSCT. No cases of breakthrough PJP infection were noted. Sixteen patients (67%) were initiated on dapsone to avoid the perceived risk of further myelosuppression with TMP-SMX and five patients (21%) because of prior delayed immune-mediated allergy to TMP-SMX. None experienced rash with dapsone therapy. Six patients (25%) were successfully rechallenged on TMP-SMX, including one patient with prior TMP-SMX-associated rash. Four (17%) patients had confirmed oxidative hemolysis, all resulting in dapsone cessation. Dapsone PJP prophylaxis in hematology patients was effective and safe, with nonlife threatening dapsone-related hemolysis noted in a small number. An absence of sulfonamide allergy cross-reactivity was noted, suggesting greater TMP-SMX rechallenges or desensitization could be considered in those receiving dapsone.