An aerosol generator such as a hand-held inhaler and method of delivering aerosol to a user inhaling on an outlet of a mouthpiece when a pressure drop is detected within the mouthpiece. A medicated fluid passing through a capillary passage is heated sufficiently to vaporize the fluid and form the aerosol by condensation of the vaporized fluid as it admixes with air. Air is supplied to the mouthpiece through an air passage which is initially closed during detection of the pressure drop. A metering chamber allows consistent delivery of precise doses of fluid to the capillary passage. The pressure drop is detected before air is supplied to the mouthpiece with the result that the aerosol can be quickly delivered to the user as the user begins to inhale on the mouthpiece. The quick delivery of aerosol provides more efficient use of the user's lung capacity.