AbstractBackgroundIsavuconazole is a triazole antifungal drug, approved for the treatment of invasive aspergillosis and mucormycosis. It has been previously reported that an interaction between flucloxacillin and voriconazole may lead to subtherapeutic voriconazole exposure, when used concomitantly. Since isavuconazole is also metabolised via cytochrome P450 enzymes, the same interaction may be expected.ObjectivesWe aim to document exposure to isavuconazole in patients concomitantly treated with flucloxacillin.PatientsWe report two patients treated with both isavuconazole and flucloxacillin, in whom we determined isavuconazole concentrations.ResultsLow isavuconazole trough concentrations (<1 mg/L) were observed in two patients under concomitant treatment with flucloxacillin.ConclusionsIn combination with flucloxacillin, low isavuconazole concentrations were observed but an adequate isavuconazole exposure may be reached with dose augmentation. Therapeutic drug monitoring of isavuconazole is recommended to ensure an adequate exposure.