A possible interaction of itraconazole, a potent inhibitor of CYP3A4, with intravenously administered methylprednisolone, was examined. In this double-blind, randomized, two-phase cross-over study, 9 healthy volunteers received either 200 mg itraconazole or matched placebo orally once a day for 4 days. On day 4, a dose of 16 mg methylprednisolone as sodium succinate was administered intravenously. Plasma concentrations of methylprednisolone, cortisol, itraconazole, and hydroxyitraconazole were determined up to 24 hr. Itraconazole increased the total area under the plasma methylprednisolone concentration-time curve (AUC(0-infinity) 2.6-fold) (P<0.001), while the AUC (12-24) of methylprednisolone was increased 12.2-fold (P<0.001). The systemic clearance of methylprednisolone during the itraconazole phase was 40% of that during the placebo phase (P<0.01). The volume of distribution of methylprednisolone was not affected by itraconazole. The mean elimination half-life of methylprednisolone was increased from 2.1+/-0.3 hr to 4.8+/-0.8 hr (P<0.001) by itraconazole. The mean morning plasma cortisol concentration during the itraconazole phase, measured 24 hr after the administration of methylprednisolone, was only about 9% of that during the placebo phase (11.0+/-9.0 ng/ml versus 117+/-49.2 ng/ml; P<0.001). In conclusion, itraconazole decreases the clearance and increases the elimination half-life of intravenously administered methylprednisolone, resulting in greatly increased exposure to methylprednisolone during the night time and in enhanced adrenal suppression. Care should be taken when itraconazole or other potent inhibitors of CYP3A4 are used concomitantly with methylprednisolone.