Systemic absorption of a drug depends on its physicochemical properties, the nature of the dosage form on which it is included and the anatomical and physiological characteristics of the site of absorption. These considerations are important on the biopharmaceutical production and evaluation of drugs: the design of the dosage forms requires a deep knowledge of the physiological and pathological factors that affect drug absorption for guarantying the therapeutic efficacy and to avoid possible drug-drug and drug-nutrient interactions.