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2. Pharmacokinetics
2.1. Absorption
Absorption refers to how medicines enter the bloodstream. A few of the most common ways to administer medicines are:
- 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.
- 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.
- 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.
- 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.
- 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.
- Intranasal: medicine is absorbed through the mucosa membranes in the nose into the bloodstream (e.g. anti-congestion spray).
- 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.