A spacer for delivering a medicinal substance to a user, the spacer comprising: - a chamber comprising, - first and second reservoirs (10, 12), - an inlet for admission of the medicinal substance into the first reservoir, - an outlet for withdrawal of the medicinal substance from the second reservoir, - a vent for expulsion of air from the second reservoir, and - a valve located at least partly within the chamber, the valve comprising, - a retaining portion which is retained by a part of the chamber, - a central first flow control portion, the first flow control portion being adapted to open during user inspiration, thereby allowing forward-flow of the medicinal substance from the first reservoir to the second reservoir, and adapted to close during user expiration, thereby substantially limiting backflow of the medicinal substance from the second reservoir to the first reservoir, and - a second flow control portion which surrounds the first flow control portion and which is anchored by the retaining portion, the second flow control portion being adapted to close during user inspiration, thereby substantially limiting inflow of air through the vent into the second reservoir, and adapted to flap open during expiration, thereby allowing expulsion of air from the second reservoir through the vent, characterised in that, - the first flow control portion forms a dome-like structure (100, 104) at rest and on exhalation by the user, the dome-like structure (100, 104) being anchored by and extending upwardly from proximate the retaining portion and defining a central cross slit proximate its peak, the central cross slit being adapted to open during user inspiration and close at rest and during user expiration, and - the second flow control portion has an outer free edge which is adapted to flap open for vent access during user expiration. - a chamber comprising, - first and second reservoirs (10, 12), - an inlet for admission of the medicinal substance into the first reservoir, - an outlet for withdrawal of the medicinal substance from the second reservoir, - a vent for expulsion of air from the second reservoir, and - first and second reservoirs (10, 12), - an inlet for admission of the medicinal substance into the first reservoir, - an outlet for withdrawal of the medicinal substance from the second reservoir, - a vent for expulsion of air from the second reservoir, - a valve located at least partly within the chamber, the valve comprising, - a retaining portion which is retained by a part of the chamber, - a central first flow control portion, the first flow control portion being adapted to open during user inspiration, thereby allowing forward-flow of the medicinal substance from the first reservoir to the second reservoir, and adapted to close during user expiration, thereby substantially limiting backflow of the medicinal substance from the second reservoir to the first reservoir, and - a second flow control portion which surrounds the first flow control portion and which is anchored by the retaining portion, the second flow control portion being adapted to close during user inspiration, thereby substantially limiting inflow of air through the vent into the second reservoir, and adapted to flap open during expiration, thereby allowing expulsion of air from the second reservoir through the vent, characterised in that, - a retaining portion which is retained by a part of the chamber, - a central first flow control portion, the first flow control portion being adapted to open during user inspiration, thereby allowing forward-flow of the medicinal substance from the first reservoir to the second reservoir, and adapted to close during user expiration, thereby substantially limiting backflow of the medicinal substance from the second reservoir to the first reservoir, and - a second flow control portion which surrounds the first flow control portion and which is anchored by the retaining portion, the second flow control portion being adapted to close during user inspiration, thereby substantially limiting inflow of air through the vent into the second reservoir, and adapted to flap open during expiration, thereby allowing expulsion of air from the second reservoir through the vent, - the first flow control portion forms a dome-like structure (100, 104) at rest and on exhalation by the user, the dome-like structure (100, 104) being anchored by and extending upwardly from proximate the retaining portion and defining a central cross slit proximate its peak, the central cross slit being adapted to open during user inspiration and close at rest and during user expiration, and - the second flow control portion has an outer free edge which is adapted to flap open for vent access during user expiration.