Rectal (PR) administration

Administration of a drug, as an enema or suppository, into the rectum. This is, for the most part, an enteral route of administration. However, venous blood from the lower rectum drains into the inferior vena cava, bypassing the liver, although upper rectal venous drainage enters the portal circulation. Therefore, a suppository pushed into the lower rectum would largely bypass first-pass metabolism and would enter the systemic circulation, while suppositories pushed further into the rectal cavity, as well as enemas, would be absorbed into both the portal and systemic circulatory systems.