1. A method for treating a bile acid dependent disease, the method comprising orally administering to a subject in need of such treatment a therapeutically effective amount of an IBAT inhibitor, wherein the IBAT inhibitor is 1,1-dioxo-3,3-dibutyl-5-phenyl-7-methylthio-8-(N—{(R)-α-[N—((S)-1-carboxypropyl)carbamoyl]-4-hydroxybenzyl}carbamoylmethoxy)-2,3,4,5-tetrahydro-1,2,5-benzothiadiazepine, or a pharmaceutically acceptable salt thereof, wherein the bile acid dependent disease comprises a defect in the intrahepatic or extrahepatic biliary tree.