The present invention relates to the inhalation delivery of aerosols containing small particles. Specifically, it relates to a method of forming an aerosol for use in inhalation therapy. The method involves: (a) heating a substrate coated with a composition comprising a drug to form a vapor, wherein the coated composition is in the form of a film less than 10μ thick; and, (b) allowing the vapor to cool, thereby forming an aerosol, which is used in inhalation therapy. In another aspect, a method of forming an aerosol for use in inhalation therapy is provided, wherein the method comprises: (a) heating a substrate coated with a composition comprising a drug to form a vapor in less than 100 milliseconds, wherein the vapor has a mass greater than 0.1 mg; and, (b) allowing the vapor to cool, thereby forming an aerosol, which is used in inhalation therapy