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 for 80-30% of the volume of air through the inhaler from a second inlet to a mouthpiece outlet , 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 , 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 for 80-30% of the volume of air through the inhaler from a second inlet to a mouthpiece outlet , 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 , 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. <EOS>