Thus, cholesterol-containing stones can be dissolved slowly. This effect can be achieved by long-term oral administration of chenodeoxycholic acid (CDCA) or ursodeoxycholic acid (UDCA). Both are physiologically occurring, stereoisomeric bile acids (position of the 7-hydroxy group being β in UCDA and α in CDCA). Normally, they represent a small proportion of the total amount of bile acid present in the body (circle diagram in A); however, this increases considerably with chronic administration because of enterohepatic cycling. Bile acids undergo almost complete reabsorption in the ileum. Small losses via the feces are made up by de novo synthesis in the liver, keeping the total amount of bile acids constant (3–5 g). Exogenous supply removes the need for de novo synthesis of bile acids. The particular acid being supplied gains an increasingly larger share of the total store. The altered composition of bile increases the capacity for cholesterol uptake. Thus, gallstones can be dissolved in the course of a 1- to 2 y treatment, provided that cholesterol stones are pure and not too large (