Working cylinder

A drug delivery system comprising:

• a reservoir from which set doses are apportioned;
• a working cylinder having a cylinder wall and open and closed ends;
• a plunger having an inner end fitting in said working cylinder, wherein said plunger extends out of said open end and defines, between said inner end and said closed end of said working cylinder, a chamber, and wherein said cylinder wall has inlet and outlet openings communicating with said chamber;
• means for connecting said inlet opening and said reservoir for fluid communication therebetween, wherein said plunger can be drawn outwardly relative to said working cylinder to draw a set dose from said reservoir into said chamber, and can be pushed back into said working cylinder to expel the set dose through said outlet opening;
• a drug delivery member mounted on said working cylinder so as to be in communication with said outlet opening;
• wherein said plunger is rotatable within said working cylinder;
• and
• wherein said plunger inner end includes a valve surface which, in a first rotational position of said plunger, blocks said outlet opening but not said inlet opening, and which, in a second rotational position of said plunger, blocks said inlet opening but not said outlet opening, whereby the rotational position of said plunger can be controlled so that said outlet opening is blocked, and said inlet opening is free, when said plunger is moved outwardly relative to said working cylinder to draw a set dose into said chamber, and said outlet opening is free, and said inlet opening is blocked by said valve surface, when the plunger is moved inwardly in said working cylinder to expel the set dose.