An inhaler comprising a reservoir of inhalable composition, an outlet valve to control the flow of composition from the reservoir, the valve outlet orifice having a maximum dimension h, measured in the direction of opening when fully opened an expansion chamber downstream of the valve having a length L and diameter D measured half way along the expansion chamber and an exit orifice at the downstream end of the expansion chamber, the orifice having a length l and a diameter d wherein 0.1
