A containment unit for a dry powder inhaler, comprising one or more containing walls which enclose and protect a dry powder from external conditions, wherein the dry powder is or contains a medicament for oral inhalation or nasal administration; at least one inlet that defines an inlet orifice in the one or more containing walls that has a fixed geometry which does not change from use of the containment unit, wherein the at least one inlet orifice has a circular cross section; at least one outlet that defines an outlet orifice in the one or more containing walls that has a fixed geometry which does not change from use of the containment unit, wherein the at least one outlet orifice has a circular cross section; wherein the at least one inlet orifice and the at least one outlet orifice are spaced apart and define a direct airflow path from the at least one inlet orifice to the at least one outlet orifice and wherein the internal diameter of the at least one inlet or outlet orifice is in the range of 0.4 to 3 mm; and an interior space within the one or more containing walls that permits the dry powder to form a powder bed that is outside the direct airflow path when the containment unit is in an orientation of use, and wherein the at least one inlet orifice and at least one outlet orifice are sized to produce secondary flows within the containment unit, whereby the at least one inlet orifice is smaller than the at least one outlet orifice one or more containing walls which enclose and protect a dry powder from external conditions, wherein the dry powder is or contains a medicament for oral inhalation or nasal administration; at least one inlet that defines an inlet orifice in the one or more containing walls that has a fixed geometry which does not change from use of the containment unit, wherein the at least one inlet orifice has a circular cross section; at least one outlet that defines an outlet orifice in the one or more containing walls that has a fixed geometry which does not change from use of the containment unit, wherein the at least one outlet orifice has a circular cross section; wherein the at least one inlet orifice and the at least one outlet orifice are spaced apart and define a direct airflow path from the at least one inlet orifice to the at least one outlet orifice and wherein the internal diameter of the at least one inlet or outlet orifice is in the range of 0.4 to 3 mm; and at least one inlet that defines an inlet orifice in the one or more containing walls that has a fixed geometry which does not change from use of the containment unit, wherein the at least one inlet orifice has a circular cross section; at least one outlet that defines an outlet orifice in the one or more containing walls that has a fixed geometry which does not change from use of the containment unit, wherein the at least one outlet orifice has a circular cross section; wherein the at least one inlet orifice and the at least one outlet orifice are spaced apart and define a direct airflow path from the at least one inlet orifice to the at least one outlet orifice and wherein the internal diameter of the at least one inlet or outlet orifice is in the range of 0.4 to 3 mm; and an interior space within the one or more containing walls that permits the dry powder to form a powder bed that is outside the direct airflow path when the containment unit is in an orientation of use, and wherein the at least one inlet orifice and at least one outlet orifice are sized to produce secondary flows within the containment unit, whereby the at least one inlet orifice is smaller than the at least one outlet orifice