An inhaler comprising a reservoir of an inhalable composition. A valve element moved by a flexible diaphragm and biased into a position in which it closes a composition flow path from a reservoir. A first air flow path is partly defined by one side of the diaphragm and a second air flow path is partly defined by the opposite side of the diaphragm. Each flow path has an outlet opening at an outlet end and the second flow path has an inlet upstream of the outlet end. The air flow paths are arranged such that suction at the outlet end causes a reduction in pressure in the first air flow path relative to the pressure in the second air flow path creating a pressure differential across the diaphragm that moves the valve element against the biasing force to open the composition flow path.