route of administration rectal route of administration http://purl.obolibrary.org/obo/NCIT_C38295 The introduction of a substance into the gastrointestinal tract by the way of the rectum, usually for systemic action. Depending on the molecular structure, drugs cross the rectal wall via either intercellular or tight junctions interconnecting the mucosal cells. Drug absorption is usually around 50% of normal oral dose. Due to the drainage pattern of the rectal veins, the hepatic first-pass effect tends to increase as the dosage form is placed deeper into the rectum. Solid suppositories represent greater than 98% of all rectal dosage forms.