The aim of this single-center, phase 1, randomized, 5 by 5 crossover, open-label study was to determine the effects of varying amounts of a nutritional supplement (Boost Plus) on the pharmacokinetics of posaconazole in 30 healthy volunteers. After an overnight fast, subjects were administered a single dose of 400 mg posaconazole oral suspension alone or following Boost Plus (8 fluid ounces [oz] [240 ml], 4 oz [120 ml], 2 oz [60 ml], or 1 oz [30 ml]). Subjects were randomized to receive all five treatments in different sequences, with a 14-day washout between treatments. Primary pharmacokinetic variables--area under the concentration-time curve from time zero to the time of the final quantifiable sample (AUC(tf)), maximum observed plasma concentration (C(max)), time to C(max) (T(max)), and relative bioavailability--were assessed up to 5 days postdose. Safety assessments included testing for adverse events, clinical laboratory tests, measurement of vital signs, physical examinations, and electrocardiograms. Posaconazole bioavailability increased almost linearly with increasing amounts of Boost Plus. Based on log-transformed data, the relative bioavailabilities (AUCs) of posaconazole were 35% (fasting), 48% (1 oz), 60% (2 oz), and 77% (4 oz) of the level reached in the presence of 8 oz Boost Plus, whereas T(max) was unaffected. Compared with the levels reached under fasting conditions, posaconazole C(max) and AUC values increased 3.5- and 2.9-fold, respectively, when given with 8 oz Boost Plus. Single doses of posaconazole at 400 mg alone and with 1, 2, 4, or 8 oz Boost Plus were safe and well tolerated in healthy subjects.