1. DRUG ADMINISTRATION ROUTES. ABSORPTION

The usage of drugs for the treatment and prevention of deseases starts with their administration into the organism or application onto the body surface. The route of administration defines the speed of onset of effect, its intensity and duration. In certain cases the route of administration determines how the medications work.

Existing administration routes are usually subdivided into enteral (via digestive tract) and parenteral (not entering into the digestive tract).

Enteral administration comprises oral, sublingual, transbuccal¹, duodenal and rectal routes.

Fig. II.1. Pharmacokinetics of drugs.

¹ From Latin bucca - cheek. A number of drugs are taken transbuccally as polymer pastilles. Their active components are absorbed via the oral mucous membrane.

The most common administration route is oral (by mouth; internally; per os). This is the most convenient and simple route of administration. Drugs do not have to be sterile to be administered this way. Absorption¹ of a number of substances (for example, acetylsalicylic acid, barbiturates and other weak electrolytes possessing an acidic nature) occurs partially in the stomach² (Fig. II.2). However, the majority of drugs are mainly absorbed in the small intestine. This is a favorable place for absorption due to the large absorbing surface of the intestinal mucous membrane (approximately 200 m²) and its intensive blood supply.

Fig. II.2. Importance of the pH of the medium for the absorption of substances from the stomach.

There are several known absorption mechanisms (Fig. II.3).

Passive diffusion occurs via the cellular membrane. It is defined by the concentration gradient of the compound. Lipophilic substances (mainly nonpolar) are easily absorbed via this route. The higher the lipophility of substances, the easier they penetrate through the cell membrane.