The rectal route of administration (ROA) is a way of administering drugs into the rectum to be absorbed by the rectum's blood vessels[Note 1] and into the body's circulatory system which distributes the drug to the body's organs and various systems[Note 2] where the drug elicits its effects.
A drug that is administered rectally will in general (depending on the drug) have a faster onset, higher bioavailability, shorter peak, and shorter duration than the oral route.
Another advantage of administering a drug rectally is that it tends to produce less nausea compared to the oral route and also prevents any amount of the drug from being lost due to emesis (vomiting, "throwing up", or "puking") since the drug is in the rectum, not the stomach, and the contents of the rectum are not lost when there is emesis.
In addition the rectal route bypasses around two thirds of the first-pass metabolism as the rectum's venous drainage is two thirds systemic (middle and inferior rectal vein) and one third portal (superior rectal vein). This means the drug will reach the circulatory system with significantly less alteration and in greater concentrations.[Note 3]
A rectal "bulb" syringe
that can be used to introduce fluids into the rectum.
The rectal ROA may be performed with either of the following:
- A suppository, a drug delivery system inserted into the rectum.
- An enema, the act of introducing a liquid-drug solution (drug(s) dissolved in water, the amount usually being less than 10 millilitres) into the rectum and sometimes the colon (usually when more liquid is used).
See also 
- ^ The rectum has numerous amounts of blood vessels available to absorbed the drug introduced into the rectum.
- ^ The organs and systems include the (depending on if the drug is able to pass the blood–brain barrier (BBB) or not) central nervous system (CNS), brain, peripheral nervous system (PNS), cardiovascular system (CVS), et cetera.
- ^ Other ROAs that bypass first-pass metabolism include inhalation (smoking, vaporizing, etc.), intravenous injection (IV), insufflation ("snorting"), et cetera, but the oral route does not bypass first-pass metabolism.
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