Rectal Medication Administration

Rectal administration places medication against the rectal mucosa as a suppository, enema, foam, or gel. It is useful when swallowing is unsafe or impossible, such as with vomiting, dysphagia, a seizure, unconsciousness, or lack of IV access. It can also deliver local treatment directly to the rectum and distal colon. The route does not produce predictable oral- or IV-equivalent exposure. The rectum has a small absorptive surface, no intestinal villi or microvilli, little residual fluid, and variable pH. Stool, formulation vehicle, contact time, and expulsion can all change the delivered dose. Absorption is therefore generally slower, more variable, and less predictable than oral or parenteral absorption. Placement affects hepatic first-pass metabolism. Drug absorbed from the lower and middle rectum drains through systemic veins toward the inferior vena cava and partly avoids the liver initially. Drug absorbed higher in the rectum drains through the superior rectal vein into the portal circulation and undergoes first-pass metabolism. Roughly half of an absorbed rectal dose may escape first pass, but the proportion changes with how high the medication is placed. Rectal administration does not...