Healthcare professionals require an easy algorithm for selecting the most appropriate inhalation product for each patient at the beginning of a treatment. As a case study, we selected five marketed formoterol products: Foradil® and Formagal®, capsule-based dry powder inhalers (DPIs), Novolizer® Formoterol and Oxis®, reservoir-based DPIs and Formoair®, a pressurized metered dose inhaler. We generated an algorithm based on device properties (i.e. device handling, feedback and remaining dose/end of product indication) and in vitro aerodynamic performances (i) along the product use life in optimal conditions, (ii) at different airflows and (iii) after exposing pre-loaded doses to 40 °C and 75% relative humidity for 4 h. Based on these results, an algorithm was built where Formoair and Formagal can be proposed when there is high risk of humidity and for patients presenting suboptimal or optimal airflows. When no risk of humidity is present, Formoair, Foradil, Formagal and Novolizer Formoterol equipped with a trigger valve could be proposed for patients presenting suboptimal airflows. When no risk of humidity is present and for patients presenting optimal airflow, all products, including Oxis, could be proposed. Ultimately, the optimal inhalation product will be selected after checking the patient's preference and capacity for correct device handling and inhalation technique.