This article reviews regulatory considerations for companies wishing to develop drugs for delivery via the respiratory tract (e.g., by oral inhalation or intranasally) using molecules previously approved for a different therapeutic indication and/or a different delivery route. Conceptually, such repurposing has many medical and business advantages, but turning promising ideas into real products requires overcoming a number of practical challenges. Obtaining regulatory approval to market a candidate product comes at the end of a product development, but being aware of the regulatory requirements is necessary before the development even starts. Understanding the scope and type of data that regulatory agencies would like to see may have a decisive influence on the design of the development program itself. For example, even for repurposed drugs, safety, efficacy, quality, and usability must be demonstrated, either from prior knowledge or through newly generated data. Furthermore, products for respiratory delivery have unique regulatory challenges due to the nature of these drug-device combinations.