Ampoules for dispensing flowable substances and metering systems incorporating the same are described herein. An ampoule for a flowable substance includes an ampoule body comprising an interior space for receiving the flowable substance and an inner circumferential surface having a guide region and a sealing region. A piston may be disposed in the interior space of the ampoule body such that the piston is slidably displaceable along a direction of displacement. The piston includes at least one sealing element for forming a seal with the inner circumferential surface in a contact zone on the outer circumference of the sealing element. The sealing element has a first compression state when the contact zone is in the guide region and a second compression state when the contact zone is in the sealing region. A second blocking device prevents movement of the contact zone from the sealing region to the guide region.