of the vial when the plunger 45 is actuated the safety device 50 illustrated in fig1 and 12 is a currently preferred embodiment of the present invention and its disk 8 constitutes the end plate of a telescopic tube 8a that has an internal end flange 8b and inside which the piston 2 is slidingly mounted a helical compression spring 51 acts between the flange 8b of the disk and the top partition 16 of the jacket 1 operation is clearly understandable from fig1 and 12 fig1 in fact illustrates the safety device in its standby or inactive position whereas fig1 is a view of the same device in the active position for the withdrawal of liquid from the line 15 in this position one of the tips of the needle 4 after passing through the stopper 24 draws from the line 15 and the other tip after piercing the stopper 14 enters a vial not shown that can be a vial 40 so as to connect the line 15 to the vial in this active position the displacement element 2 abuts against the connector 5 whereas the end plate 8 abuts against the displacement element and remains temporarily coupled to it by virtue of the presence of snap action engagement means which are constituted by a first temporary engagement means for example constituted by an annular ridge 82 formed on the piston like element that can engage with second temporary engagement means on the abutment element.