Plasma levels of omeprazole (OPZ) in Japanese male subjects were compared after a single oral administration of 20 mg of OPZ enteric-coated tablets with and without coadministration of Maalox (MLX suspension) or Maalox dry suspension granules (MLX granules). After coadministration of MLX granules, plasma levels of OPZ markedly decreased, and area under the blood concentration-time curve (AUC) decreased to 26% of that of OPZ alone. In contrast, only a slight decrease in AUC was observed after coadministration of OPZ and MLX suspension. Both MLX suspension and MLX granules exhibited similar degrees of the inhibitory effect on the renal excretion of levofloxacin. It was suggested that a specific and unexpected drug interaction occurred between OPZ enteric-coated tablets and MLX granules via a distinct mechanism from that reported for fluoroquinolones and MLX suspension.