Demonstrating bioequivalence (BE) for complex generics such as nasal spray suspension drug products is a challenging task. Sponsors not only have to consider device requirements but also understand the properties of the API in the presence of functional excipients. The process of achieving equivalence to the reference listed drug product is therefore complex. In this presentation we explore the requirements for BE and the application of in vitro techniques for API and formulation characteristics. A systematic approach for measuring and assessing the API particle size changes within the formulation will be discussed, alongside the link these measurements have to API solubility and bioavailability. We also explore how the formulation and device interact through considering the force required to actuate during dose delivery. Together, these analytical methods facilitate the determination of critical material and process attributes that may affect drug product quality. As part of the presentation we will reference research carried out during the development of a generic mometasone furoate product.