Combining voriconazole and flucloxacillin is indicated in patient cohorts experiencing both invasive aspergillosis and Gram-positive infections (e.g., patients with chronic granulomatous disease or postinfluenza pulmonary aspergillosis). We report a highly relevant interaction between voriconazole and flucloxacillin, resulting in subtherapeutic plasma voriconazole concentrations in more than 50% of patients, that poses a severe threat if not managed properly.KEYWORDS aspergillosis, drug interactions, exposure, influenza, pharmacokinetics V oriconazole is a triazole antifungal that is used to treat a broad range of fungal infections (1). Its use is complicated by nonlinear pharmacokinetics and drug-drug interactions (2). Voriconazole may act as an inhibitor or as a substrate of the enzymes cytochrome P450 2C9 (CYP2C9), CYP2C19, and CYP3A4 and thus may alter its own pharmacokinetic profile and that of coadministered drugs (2). Many interactions have been well documented and can be managed through dose adjustments and close therapeutic drug monitoring of voriconazole, relevant coadministered drugs, or both (2).However, more than once, drug-drug interactions have been discovered in daily practice in addition to those found during drug development. Here, we report a highly relevant interaction of voriconazole-flucloxacillin that resulted in suboptimal or even undetectable concentrations of voriconazole.As concomitant voriconazole and flucloxacillin therapy is given to specific patient groups, such as those with chronic granulomatous disease or influenza-associated aspergillosis, this might be a threat to adequate antifungal treatment.Patients were identified in five Dutch tertiary-care university hospitals from June 2014 to June 2016. Initially, three patients were identified on the basis of undetectable plasma voriconazole concentrations. This prompted us to search electronic patient records to retrospectively identify additional cases of the concomitant use of voriconazole and flucloxacillin. The data collected were gender, age, underlying disease, duration of voriconazole therapy, indication, voriconazole dose, plasma voriconazole and