rectal delivery

Rectal delivery is administering a drug into the rectum — by suppository, enema, or rectal gel — either to treat something local (haemorrhoids, constipation, inflammation) or to get a drug into the body when the oral route is unavailable. It is the practical back-up when a patient is vomiting, unconscious, or unable to swallow.

The rectum has a moist, well-perfused lining that can absorb many drugs reasonably well. An interesting quirk of its anatomy is partial first-pass avoidance: blood from the lower rectum drains into veins that bypass the liver, while blood from the upper rectum goes through the portal vein to the liver. So a drug placed low in the rectum can escape some, though not all, first-pass metabolism — the exact fraction is variable and hard to control. Local action, by contrast, simply needs the drug to stay and act on the rectal tissue.

The route is valuable for infants, for nausea and vomiting (antiemetics), for seizures (rectal diazepam), and for local bowel disease. Its drawbacks are erratic and incomplete absorption, the limited and sometimes unpredictable surface contact, possible expulsion of the dose, and — importantly — low patient acceptability in many cultures, which limits how often it is chosen when alternatives exist.