The terms drug substance (DS) and active pharmaceutical ingredient (API) are often used interchangeably, although some regulatory and manufacturing frameworks distinguish between them. A drug substance generally refers to the active material after it has undergone full synthesis, purification, and characterisation, whereas the API is the component within a finished drug product responsible for therapeutic activity.
DS and API are considered equivalent under ICH Q7 definitions. DS is used in a regulatory and manufacturing context, typically addressed in Drug Master Files (DMFs), GMP documentation, and ICH guidelines. The focus is on its manufacture, purification, characterisation, and control prior to formulation into the final drug product. API is used in regulatory submissions, such as NDAs and ANDAs, product labelling, and clinical documentation. API is considered in terms of its role, quality, and performance in the final formulated drug product.