ABSTRACT
This chapter describes the five processes that feature in drug absorption into the body: passive diffusion down a concentration gradient, the cell membrane and fat-solubility of drugs, active transport, disintegration and dissolution of tablets, and presystemic metabolism. Modified-release drugs are used when the duration of action of a short-acting drug needs to be prolonged, as with the heart drug nifedipine. Presystemic metabolism takes place before the drug reaches the systemic circulation. The absorbing surface of the small intestine has a mechanism to detoxify toxins in food. The main principles of drug absorption are: the disintegration and dissolution of tablets, passive diffusion down a concentration gradient, the cell membrane and fat-solubility of drugs, active transport of ions and water-soluble drugs and presystemic metabolism. As might be expected, the body treats most drugs as toxins, and a proportion of many commonly used drugs are inactivated in the intestinal villi, before absorption can occur.
