A device for dispensing a liquid medium (26) from a container (1), comprising a housing (13), a metering drum (2) accommodated therein and a sealing disk (14), the housing (13) having a drum outlet (3) and a system air admission structure (22), and the sealing disk (14) having an admission opening (12) and a drum inlet (4). The metering drum (2) comprises a metering chamber (5), a chamber outlet (6) and a chamber inlet (7). The metering drum (2) further has a rotary valve (8), said rotary valve (8), in a first working position, connecting the drum inlet (4) to the chamber inlet (7) while closing the chamber outlet (6), thereby admitting air to the container (1) and removing air from the metering chamber (5). In a second working position, The metering drum (2) closes the chamber inlet (7) while connecting the drum outlet (3) to the chamber outlet (6), thereby further admitting air to the metering chamber (5) and the air admission channel (11) being connected to the system air admission structure (22).