The large intestine is responsible for the conservation of water and electrolytes, formation of a solid stool, and storage of faeces until a convenient time for defaecation. Its function is quite distinct from that of the small intestine whose primary role is the digestion of food and absorption of simple nutrients.
Administration by the rectal route is preferable for drugs which produce emesis or are irritant when given orally. For the purposes of drug delivery, the colon has to be considered as two regions; the distal colon, which can be reached from the anus, and the proximal colon, which is only accessible via the oral route. The splenic flexure limits the area of exposure of drugs administered by the anal route to the descending and sigmoid colon, rectum and anus. Instillation of large volumes to overcome this restriction triggers the defaecation reflex. Nevertheless the rectal route is popular in Europe, though not in the U.S.A! Formulations targeted to the proximal colon have to be delivered via the oral route and must be protected against the hostile environment of the stomach and small intestine. Transit through the colon is slower than other areas of the gastrointestinal tract and so there is an opportunity for sustained drug delivery from the ascending and first part of the transverse colon.
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