An inhalation system, comprising: a dry powder formulation comprising a plurality of powder particles of a diketopiperazine; and a breath powered dry powder inhaler provided with relatively rigid air conduits and configured to deliver the particles to the pulmonary circulation of a subject at a single inhalation, portions of the inhaler defining a first flow path for 20-70% of the volume of air through the inhaler, said path leading from a first inlet of a container through said container to at least one dispensing port communicating with a second flow path, said container being configured to hold the powder and being configured so that air entering the container re-circulates within the internal volume of the container and entrains the particles in the air flow, said at least one dispensing port having a cross-sectional area less than the first inlet and providing a size exclusion aperture having a smallest dimension between 0.25 mm and 3 mm for particles exiting the container, whereby particles that pass through said at least one dispensing port immediately accelerate; portions of the inhaler defining the second flow path (340, 940) for 80-30% of the volume of air through the inhaler from a second inlet (310, 910) to a mouthpiece outlet (335, 935), said second flow path by-passing said first inlet to said container and merging with the first flow path as said first flow path leaves said container via said at least one dispensing port; the first flow path changing in direction through approximately 90° to the axis of the dispensing port where it merges with the second flow path (340, 940), fluidized particles exiting the container via said at least one dispensing port being sheared on contact with the flow in the second flow path; wherein the inhalation system is operably configured to emit a powder plume comprising diketopiperazine microparticles having a volumetric median geometric diameter measurable by laser diffraction ranging from 2μm to 8μm and a geometric standard deviation of less than 4 μm. a dry powder formulation comprising a plurality of powder particles of a diketopiperazine; and a breath powered dry powder inhaler provided with relatively rigid air conduits and configured to deliver the particles to the pulmonary circulation of a subject at a single inhalation, portions of the inhaler defining a first flow path for 20-70% of the volume of air through the inhaler, said path leading from a first inlet of a container through said container to at least one dispensing port communicating with a second flow path, said container being configured to hold the powder and being configured so that air entering the container re-circulates within the internal volume of the container and entrains the particles in the air flow, said at least one dispensing port having a cross-sectional area less than the first inlet and providing a size exclusion aperture having a smallest dimension between 0.25 mm and 3 mm for particles exiting the container, whereby particles that pass through said at least one dispensing port immediately accelerate; portions of the inhaler defining the second flow path (340, 940) for 80-30% of the volume of air through the inhaler from a second inlet (310, 910) to a mouthpiece outlet (335, 935), said second flow path by-passing said first inlet to said container and merging with the first flow path as said first flow path leaves said container via said at least one dispensing port; the first flow path changing in direction through approximately 90° to the axis of the dispensing port where it merges with the second flow path (340, 940), fluidized particles exiting the container via said at least one dispensing port being sheared on contact with the flow in the second flow path; wherein the inhalation system is operably configured to emit a powder plume comprising diketopiperazine microparticles having a volumetric median geometric diameter measurable by laser diffraction ranging from 2μm to 8μm and a geometric standard deviation of less than 4 μm.