Drug formulation can be defined as the process in which different chemical substances, including the active drug, are combined to produce a final medicinal product. Drug substances are seldom administered alone but rather they are given as part of a formulation in combination with one or more agents that serve varied and specialised pharmaceutical functions. These non-medicinal agents (pharmaceutical ingredients or excipients), are used to fashion medications into acceptable dosage forms. This chapter discusses the various topics related to drug formulation including excipients and their adverse effects. An understanding of these concepts provides the practitioner with an appreciation of why different formulations of medication are needed, as well as an insight into the consequences of altering the formulation for a recipient.