Combination products are complex by definition, and the product development cycles can be both time- and resource-intensive. The common theme of challenges has been formulation device incompatibility, lack of in vitro bioequivalence and the expensive in vivo failures. This article by Linda Liao of Proveris Scientific offers an instrumental guide to developing nasal spray and pressurised metered dose inhaler (pMDI) products for both innovator and generic drug products, using a quality by design approach.