The present invention addresses the problem of turbophoresis and provides a relatively simple and low cost solution for a metered dose inhaler, in particular, a pressure-actuated metered dose inhaler. A metering valve (100) comprises a valve body (110), a valve stem (130) mounted within the valve body (110) and a spring element (140) for biasing the valve stem (130) in an “at rest” position. A pre-metering region (170) is formed in the valve stem (130) which is provided with a miniature valve seal (210) at its inlet (180). The miniature valve seal (210) effectively isolates the pre-metering region (170) from surrounding bulk formulation (4) within a canister (1) in which the metering valve (100) is mounted when the valve stem (130) is in the “at rest” position.