We will deal here with the process of drug absorption from different sites in the body. Drug absorption, whether it be from the gastro-intestinal tract or from the buccal mucosa, requires the passage of the drug in a molecular form across the barrier membrane. Most drugs are presented to the body as solid or semisolid dosage forms and these must first release their drug content. Drug must then dissolve and, if it has the appropriate physical properties, will pass from a region of high concentration to a region of low concentration across the membrane surrounding the site of absorption into the blood (figure 9.1).