Intravenous

This is the most common route employed in the critically ill. It is reliable, having no problems of absorption, avoids first-pass metabolism and has a rapid onset of action. Its disadvantages include the increased risk of serious side-effects and the possibility of phlebitis or tissue necrosis if extravasation occurs.

Intramuscular

The need for frequent, painful injections, the presence of a coagulopathy (risk the development of a haematoma, which may become infected) and the lack of muscle bulk often seen in the critically ill means that this route is seldom used in the critically ill. Furthermore, variable absorption because of changes in cardiac output and blood flow to muscles, posture and site of injection makes absorption unpredictable.

Subcutaneous

Rarely used, except for low molecular weight heparin when used for prophylaxis against DVT. Absorption is variable and unreliable.

Oral

In the critically ill this route includes administrations via NG, NJ, PEG, PEJ or surgical jejunostomy feeding tubes. Medications given via these enteral feeding tubes should be liquid or finely crushed, dissolved in water. Rinsing should take place before and after feed or medication has been administered, using 20–30 ml WFI. In the seriously ill patient this route is not commonly used to give drugs.