Abstract: A system for dispensing a clinically effective dose of an inhalable, pharmaceutically active compound, which system includes a dry powder inhaler device containing a powder containing the pharmaceutically active compound, wherein (a) of the powder particles dispensed by the inhaler device (the “metered DPI dose”), at least 40% are less than about 10 microns in diameter at the point the particles exit the inhaler, (b) the metered DPI dose is sufficient to produce a clinically effective result in a patient; and (c) the amount of the compound in the metered DPI dose is not more than 70% of the minimal amount of the compound which, when dispensed in a pressurized metered dose inhaler, produces an equivalent clinically effective result in the same patient.