ABSTRACT: BACKGROUND: Patients in intensive care units receive many drugs simultaneously but through limited venous accesses. Several intravenous therapies have to be administered through the same catheter, thus increasing the risk of physicochemical incompatibility. The purpose of this work was to assess and to quantify the impact of physical incompatibility on the mass flow rates of drugs infused simultaneously to the patient, through an in vitro study. METHODS: Furosemide-midazolam incompatibility was used to assess the impact of physical incompatibility on drug mass flow rates. Furosemide, midazolam, and saline were simultaneously infused. A filter was added at the end of the infusion line to retain visible particles. Two infusion conditions were tested with and without visible particles. A partial least square method on UV spectra was used to determine simultaneously the concentrations of the two drugs at the egress of the terminal extension line. The drug mass flow rate (expressed as mg/h) was calculated as the product of drug concentration versus total flow rate. Observed/theoretical mass flow rate ratios for each drug (%) were determined per infusion condition. RESULTS: Even in the absence of visible particles, precipitation of furosemide led to a drug loss estimated at between 10 % and 15 %. Furosemide is more impacted by interaction because the pH of the mixture is acid and this form is poorly soluble in an aqueous solution. CONCLUSIONS: Physical incompatibility between furosemide and midazolam leads to a significant reduction in drug delivered to the patient and may result in treatment failure.