last updated 23-05-2024

Routes of administration

Routes of administration

This is first step for getting the drug into the body. Each route of administration has its advantages and disadvantages. The most important concept is to understand how the plasma concentration of a drug versus time profile differs depending on the route of administration. Intravenous (IV) and oral administration (PO) are the routes that are most extensively described and used for PK studies in humans. These administration routes are discussed extensively in the upcoming pages. There are, however, many alternative routes. Five common routes are briefly described here:


Subcutaneous (SC)

Injection just under the skin. Commonly used for insulin injection. Absorption can be fast from aqueous solution but slower with depot formulations. Absorption is usually complete. Improved by massage or heat.


Intramuscular (IM)

Absorption can be rapid from aqueous solution. Absorption is sometimes erratic, especially for poorly soluble drugs, e.g. diazepam, phenytoin. Irritating drug may be painful.



May be used for a local effect, e.g. bronchodilators. Can be used for systemic effect, e.g. general anaesthesia. Rapid absorption by-passing the liver. Absorption of gases is relatively efficient.


Topical or transdermal administration

Mostly local effect - ear drops, eye drops, creams and ointments. Generally absorption is quite slow. (beware of skin lesions!). Occlusion may improve absorption. Eye drops have a primarily local effect, but can absorbed via lacrimal ducts. Transdermal patches can provide prolonged or controlled drug delivery (steroids). Systemic absorption is better with low dose, low MW, and lipid soluble drugs.


Rectal (PR)

Drugs given by the rectal route are most commonly given as suppository and include paracetamol, chlorpromazine and some barbiturates. Absorption may be improved over oral route as some (but not all) of the veins draining the rectum lead directly to the general circulation by-passing the liver. Thereby reducing the first-pass effect. This route may be most useful for patients unable to take drugs orally or with younger children.


A patient with post-operative pain is treated with morphine. Morphine can be administrated in different forms. What form of administration is associated with the lowest Tmax , when morphine is administrated in a similar dose?