A medical device (1) for percutaneous or venous access for the purposes of administering a fluid to a patient or sampling from one, comprising a body (2) from which there emerges a cannula (4) supported at one distal end (7) of a cannula holder (8), a proximal end (9) of said cannula holder (8) being connected to a tube (10) in which said fluid can circulate, said cannula holder (8) being capable of moving within said body (2) of the device (1) under the force of a resilient thrust element (26), an immobilising member (32) of one piece with said body (2) and capable of impeding said movement of the cannula holder (8) and being provided with means (24) for activating such movement associated with the body (2) of the device (1) acting together with counter-means (35) of one piece with said cannula holder (8). Said cannula holder (8) has a part or portion of cross-section which is reduced or different (80) from its said distal end (7) and said proximal end (9), said part or portion (80) of reduced or different cross-section allowing said cannula holder (8) to deform within the body (2) of the device (1) when said activating means are activated and such as to separate said cannula holder from said immobilising member (32) and allowing the cannula holder (8) to move in this body (2) of the device (1) and re-enter and disappear within such body (2) of the cannula (4).
