The therapeutical use of drugs involves the application of dosage forms, serving as carrier systems together with several excipients to deliver the active ingredient to the site of action. Drug delivery technology combines an understanding of medicinal chemistry and pharmacology with the skill of formulation, aiming the preparation of improved pharmaceuticals. The recently introduced Biopharmaceutical Classification System provides guidance for dosage form design, taking the molecular and physico-chemical properties of drugs into consideration through their solubility and permeability characteristics. Pharmaceutical excipients used for oral dosage form have been traditionally assumed as being inert. However, recent experience and new results have shown that they can interact with the active drug ingredient, affecting its dissolution, absorption and bioavailability. Classification of the excipients is based on their role in the pharmaceutical formulation and on their interactions influencing drug delivery, based on their chemical and physico-chemical properties. The main classes are the antioxidants, coating materials, emulgents, taste- and smell-improvers, ointment bases, conserving agents, consistency-improvers and disintegrating materials. Some of the excipients may serve multiple purposes; for example, methylcellulose is a coating material, is applied in the preparation of suspensions, to increase viscosity, as a disintegrating agent or binder in tablets. The aim of this paper is to review the drug-excipients with respect to their chemistry, importance and interactions altering the pharmacokinetics of the drug substances. Emphasis will be given to two major classes of excipients: the antioxidants and disintegrants (substances facilitating disintegration of the drug tablets in the gastro-intestinal tract). Details will be given on the mechanisms through which they can alter drug effectiveness and tolerance, and control their application. Examples and references will be given for their analysis.