A medical device for percutaneous or venous access for the purposes of administering a fluid to a patient or sampling from one is disclosed. The device includes a cannula holder that has a part or portion of cross-section which is reduced or different from its distal end and proximal end. The part or portion of reduced or different cross-section allows the cannula holder to deform within the body of the device when activating means are activated to separate the cannula holder from an immobilising member allowing the cannula holder to move in the body of the device and re-enter and disappear within such body of the cannula.