A metered dose inhaler for use with a pressurized aerosol container which is preferably breath-actuated. A preload is applied to the internal aerosol valve by an amount sufficient to result in a dose release, but this is prevented by the application of a pneumatic resisting force. The inhaler comprises a release device which, upon actuation, releases the resisting force and allows the preload to actuate the aerosol valve. A metered dose of medicament is then released for inhalation by the patient. The pneumatic resisting force is established by a negative pressure region defined in part by a diaphragm. The diaphragm includes a central disk of a first, relatively high stiffness material and a peripheral ring, coupled by a flexure of a second, relatively low stiffness material.