Background: Easyhaler (registered trademark by Orion Corporation) is a multidose dry powder inhaler (DPI) for the treatment of asthma and chronic obstructive pulmonary disease (COPD), designed to be simple and easy to use. Salmeterol-fluticasone propionate (S-F) Easyhaler (50/250 and 50/500 lg per dose), available in several European countries, provides combined inhaled corticosteroid and long-acting beta agonist therapy for the management of asthma and COPD. A requirement of the European Committee for Medical Products for Human Use guidelines is to demonstrate product performance under conditions that mimic real-life patient use. Therefore, our aims were to assess the robustness of the S-F Easyhaler by assessing the delivered dose (DD) and fine particle dose (FPD) throughout the inhaler lifespan and under simulated environmental stress conditions. Methods: This was a noncomparative exploratory in vitro study. Two batches and six to nine inhalers per batch from both dose strengths were used to assess drug delivery performance over the inhaler lifespan (doses 1-60). For determining the impact of simulated environmental stress (tests for exposure of dropping, vibration, moisture, and freeze-thawing) on DD and FPD, one batch and three inhalers per batch from both dose strengths were used per test, respectively. Aerodynamic particle size distribution was evaluated during the simulated dropping and vibration tests. Results: DD and FPD from both dose strengths of S-F Easyhaler performance remained consistent through the inhaler lifespan and simulated environmental stress did not affect its performance. Similar DD and FPD values were observed with or without dropping, vibration, exposure to moisture, and freeze-thawing, and no inhaler breakages occurred during the simulated tests. Conclusions: The in vitro performance of S-F Easyhaler at both dose strengths suggests that reliable dosing and robustness can be achieved under real-life stress conditions; S-F Easyhaler is a durable DPI for the management of asthma and COPD.