Patients receiving nonsteroidal anti-inflammatory drug therapy may also require administration of corticosteroids, particularly patients with rheumatoid arthritis. To investigate the effect of rofecoxib on the single-dose pharmacokinetics of oral prednisone and intravenous prednisolone, the authors conducted a randomized, double-blind, placebo-controlled crossover study in 12 healthy subjects. Oral rofecoxib (250.0 mg/day for 14 days) failed to influence prednisone or prednisolone pharmacokinetics after intravenous prednisolone or oral prednisone administration. The geometric mean ratio (GMR) (90% confidence interval) of prednisolone AUC infinity (rofecoxib/placebo) following intravenous and oral corticosteroid was 0.97 (0.94, 1.01) and 0.99 (0.91, 1.08), respectively. Similarly, the prednisone AUC infinity GMRs (rofecoxib/placebo) after intravenous and oral corticosteroid were 1.03 (0.95, 1.11) and 1.08 (0.92, 1.28), respectively. The absence of an effect of rofecoxib on the pharmacokinetics of oral prednisone or intravenous prednisolone indicates that no adjustment in dose of this corticosteroid is necessary when administered concurrently with rofecoxib.