Key Principles of Pharmacology


2. Pharmacokinetics

2.1. Absorption

Absorption refers to how medicines enter the bloodstream. A few of the most common ways to administer medicines are:

  1. Oral administration (PO: per os – by mouth) is a route where a substance (e.g. tablet, syrup) is taken through the mouth to be absorbed from the stomach or intestine, enters the liver and the general circulation. This method is used in 80% of the medicines used in medical practice because the route is convenient and economical.

  2. Sublingual (SL), where the medicine is dissolved under the tongue (e.g. sublingual tablets). and is absorbed through mucous membranes into the bloodstream. This route is fast and convenient.

  3. Transdermal (TD): medicine is contained in a patch, and absorbed through the skin (e.g. nicotine, oestrogen). The major advantage for this route is the medicine dosage is continuous and long-acting; however, it may cause local irritation due to the adhesive patch on the skin.

  4. Rectal (PR), where medicine (e.g. analgesic and anti-nausea suppositories) is inserted into the rectum; the medicine is absorbed through mucosa into the bloodstream. It is useful for unconscious or vomiting patients, but the medicine may be incompletely or irregularly absorbed.

  5. Inhalation: medicine is inhaled as a gas, aerosol or mist (e.g. bronchodilator). The medicine is usually intended to act directly on lung tissue. This route produces a rapid onset of action, but the inhaled medicine may irritate lung tissue.

  6. Intranasal: medicine is absorbed through the mucosa membranes in the nose into the bloodstream (e.g. anti-congestion spray).

  7. Intravenous - IV, intramuscular - IM, subcutaneous – SQ or SC (all of these are known as parenteral). This route produces a faster response than oral or rectal routes and avoids unpredictable absorption through the gastrointestinal system; however, it requires sterile conditions to prevent infections.