rectal administration

Rectal administration delivers a drug into the rectum, usually as a suppository (a small solid that melts at body temperature) or an enema (a liquid). It is a useful back-up when the mouth route is impossible — for instance in a vomiting child, an unconscious patient, or someone who cannot swallow.

The rectum has a good blood supply, and the veins draining its lower part empty partly into the general circulation rather than into the portal vein to the liver. This means a drug absorbed from the lower rectum partly bypasses first-pass metabolism, so more active drug can reach the body than from an equivalent swallowed dose. However, the degree of bypass is variable and unpredictable, because drainage from the upper rectum still goes to the liver and the drug can mix between regions.

Rectal dosing can be used both for local effects (such as treating haemorrhoids or inflammation of the lower bowel) and for systemic effects (such as fever reduction or anti-seizure medication). It avoids the destructive enzymes and acid of the stomach.

The limitations are mainly absorption that is often slow, incomplete, and erratic, the irritation a suppository can cause, and the simple fact that the route is socially awkward and disliked by many patients — so it tends to be reserved for situations where the oral route truly cannot be used.