Absorption

Drugs may be given by a variety of routes; the route chosen depends on the desired site of action and the type of drug preparations available. Routes used commonly by the anaesthetist include inhalation, intravenous, oral, intramuscular, rectal, epidural and intrathecal. Other routes, such as transdermal, subcutaneous and sublingual, also can be used. The rate and extent of absorption after a particular route of administration depends on both drug and patient factors.

Drugs may be given orally for local as well as systemic effects, for example, oral vancomycin used to treat pseudomembranous colitis is acting locally; antacids also act locally in the stomach. In such cases, systemic absorption may result in unwanted side effects.

Intravenous administration provides a direct, and therefore more reliable, route of systemic drug delivery. No absorption is required, so plasma levels are independent of such factors as gastrointestinal (GI) absorption and adequate skin or muscle perfusion. However, there are disadvantages in using this route. Pharmacological preparations for intravenous therapy are generally more expensive than the corresponding oral medications, and the initially high plasma level achieved with some drugs may cause undesirable side effects. In addition, if central venous access is used, this carries its own risks. Nevertheless, most drugs used in intensive care are given by intravenous infusion this way.

Oral

After oral administration, absorption must take place through the gut mucosa. For drugs without specific transport mechanisms, only unionized drugs pass readily through the lipid membranes of the gut.