01.07 Rectal

The rectum is at the end of the large intestine, and is about 15-20 cm long. Drugs are specially formulated for rectal administration, and the Solid Drug Formulations for Rectal Use are known as suppositories.

Rectal administration avoids first pass liver metabolism. This is because, after insertion into the lower part of the rectum, absorption is into rectal veins and then into the general circulation. As the absorption is not into the portal vein, this avoids first pass metabolism. Another advantage of rectal administration is that the rectum has little enzymatic activity, and this route avoids upper gastro-intestinal acids and enzymes, which destroy many drugs. Rectal administration may be useful in the presence of upper gastrointestinal disease, which affects the absorption of the drug given via oral administration.

Rectal administration can be useful when oral administration is not possible. For instance, in nausea and vomiting, or in subjects who have difficulty in swallowing, oral administration is not possible. Post-operative vomiting prevents oral administration, but drugs can be administered rectally to reach the circulation to be effective. The antiemetic Prochlorperazine can be used rectally when nausea and vomiting are preventing oral administration.

In children in status epilepticus, it is not possible to get the child to swallow, but rectal diazepam can be used to control the epilepsy. Rectal administration can be by carer or paramedic.

In addition to being used to achieve systemic effects, the rectal route can also be used to give a local effect. For example, suppositories can be used in the treatment of haemorrhoids.

Suppositories and enemas are different. Enemas contain hypertonic solutions that draw water to them to produce a laxative effect, and are used in subjects with constipation.

The major disadvantage of rectal administration is that it is not popular with people.