Ritonavir (Norvir) is a well-known inhibitor of CYP 3A4, and most of the CYP liver enzymes. Initially developed for treating HIV, it became well known as a CYP inhibitor and is currently used in conjunction with other drugs to maintain or raise their concentrations in the body. It is a very large small molecule that breaks most of Lipinski's Rule of 5 ADME conditions. It has mass of 720 Da and 18 rotatable bonds.