A system and method for determining a minimum effective dose of an inhaled drug for an individual patient at a given time. The system includes an apparatus to measure a performance measurement of the patient and a data processor capable of communication with the apparatus. The data processor provides a computed minimum effective dose for the individual patient by applying a predetermined rule the performance measurement. The method includes measuring a performance measurement of the patient, communicating the performance measurement to a data processor and computing a minimum effective dose for the patient by applying a predetermined rule to the performance measurement within the data processor.