Background: Generally capsule-based dry powder inhalers (DPIs) are required to be used at a flow rate of 60 L/min to initiate removal and deaggregation of the drug from carrier for effective pulmonary deposition. This may not be appropriate for children or those with lung diseases. In addition, capsule-based DPI are intended for treatment over a 4-week period and storage conditions can influence the aerosolisation drug deposition, reproducibility of inhalation dose and treatment outcome. Methods: Inhalation grade lactose was blended with micronized salbutamol (50:1w/w) and filled (20±1mg) in to size 3 hypromellose inhalation grade capsules stored at 22ºC 40% RH for 4 weeks. Samples were tested using a 2-pin inhaler and aerosolised through a next generation impactor at flow rates of 30 L/min (actuated for 8s) and 60 L/min (actuated for 4s) at weekly intervals for 4 weeks. Deposition of drug in the capsule, device and emitted dose (ED), fine particle dose (FPD), percentage fine particle fraction (FPF) and mass aerodynamic diameter (MMAD) were calculated. Results: Powder retention within capsules was higher at 60 L/min whereas it was higher in the device at 30 L/min (p<0.05/Tukey). The ED, FPD, FPF was significantly greater at 60 L/min compared to 30 L/min at each time point (p<0.05/Tukey). However, there was no significant difference comparing each flow rate over time. Conclusion: Differences in results between the air flow rates at each weekly time points highlight the important relationship between inhalation, therapeutic dose, lung deposition and potential therapeutic outcome.