An inhaler comprises a tubular body in which an air flow passage is defined. A single dose of an active, inhalable, particulate substance is arranged within the air flow passage and is sealed or closed in relation to the ambient atmosphere by closure means, such as removable caps, a section of the flow passage extending from a free mouthpiece end of the tubular body along a major part of the total length of the flow passage is preferable 7-35 mm.sup.2, for example about 20 mm.sup.2. The inhaler may be adapted to be used only once, and the tubular body of the inhaler may be a length of a simple tube similar to a drinking straw.