An in-line droplet delivery device and related methods for delivering precise and repeatable dosages to a subject for pulmonary use is disclosed. The in-line droplet delivery device includes a housing, a mouthpiece, a reservoir, an ejector mechanism, and at least one differential pressure sensor. The in-line droplet delivery device is automatically breath actuated by the user when the differential pressure sensor senses a predetermined pressure change within housing. The in-line droplet delivery device is then actuated to generate a plume of droplets having an average ejected particle diameter within the respirable size range, e.g, less than about 5-6 μm, so as to target the pulmonary system of the user, the droplet delivery device is configured in an in-line orientation in that the housing, its internal components, and various device components (e.g., the mouthpiece, air inlet flow element, etc.) are orientated in a substantially in-line or parallel configuration (e.g., along the airflow path) so as to form a small, hand-held device.