Introduction: It is important to investigate the particle-particle interactions taking place in DPI formulations. These are cohesive (drug-drug), and adhesive (drug-carrier) interactions that play an important role in DPI performance during downstream processing such as capsule filling. Understanding such interactions helps in predicting problems such as aggregation or powder segregation, and controlling drug product quality. Aim: Understanding interparticulate interactions to predict DPI behaviour during high speed capsule filling for ensuring uniformity of weight and content. Materials and Methods: Three different powder blends of either a bronchodilator (X) or an ICS, Inhaled Corticosteroid (Y) or both (X plus Y) were manufactured with inhalation grade lactose as a carrier. All blends were characterized by dynamic testing (Basic Flowability Energy and Specific Energy), bulk measurements (Permeability and Compressibility), shear testing (Cohesion/Adhesion). Capsules of the three blends were filled by a high speed capsule machine dosator type. Fill weights and content uniformity were assessed. Results and Discussion: Capsule fill weight achieved was consistent for all blends, however slightly different content uniformity at higher filling speeds was found, which can be related to different drug-drug particle interactions and drug-carrier particle interactions. Conclusion: Similar DPI blends could react differently depending on the particle-particle interactions, therefore applying dynamic and bulk measurements as a characterization tool is valuable in understanding the behaviour of these blends in downstream process steps, and selecting the appropriate processing conditions accordingly to ensure that quality and robustness are built-in