Medical device for the injection of a fluid into a patient's body, said device comprising: - an injector, comprising a reservoir of the fluid to be injected, - a needle comprising a free end for the delivery of the fluid and being fluidically connected to said injector, - a hollow tubular main body extending according to a longitudinal development direction and configured to house said needle inside thereof, said main body comprising means for adjusting the position of said needle with respect to said main body, which means for adjusting the position is configured to engage with respective guide means of said needle and selectively operable to allow a sliding of said needle with respect to said main body according to a direction parallel to, or coinciding with, said longitudinal development direction, in such a way as to increase/decrease the length of the portion of needle exiting with respect to said main body, wherein said guide means comprises a second guide element at a rear end of said needle opposite to said free end, said second guide element having a prismatic profile and wherein said main body comprises a support portion coupled to said means for adjusting the position, wherein said support portion comprises a prismatic seat having a geometry complementary to that of said second guide element for housing the latter inside thereof, wherein said means for adjusting the position comprises a ring nut element which comprises a threaded hole configured to engage with said guide means, and said guide means comprises a first guide element configured as a threaded element, keyed onto said needle at a rear end thereof opposite to said free end, wherein said first guide element is configured to engage with said threaded hole of said ring nut, wherein said needle comprising said first and second guide elements (18, 20) is made of a single piece. - an injector, comprising a reservoir of the fluid to be injected, - a needle comprising a free end for the delivery of the fluid and being fluidically connected to said injector, - a hollow tubular main body extending according to a longitudinal development direction and configured to house said needle inside thereof, said main body comprising means for adjusting the position of said needle with respect to said main body, which means for adjusting the position is configured to engage with respective guide means of said needle and selectively operable to allow a sliding of said needle with respect to said main body according to a direction parallel to, or coinciding with, said longitudinal development direction, in such a way as to increase/decrease the length of the portion of needle exiting with respect to said main body, wherein said guide means comprises a second guide element at a rear end of said needle opposite to said free end, said second guide element having a prismatic profile and wherein said main body comprises a support portion coupled to said means for adjusting the position, wherein said support portion comprises a prismatic seat having a geometry complementary to that of said second guide element for housing the latter inside thereof, wherein said means for adjusting the position comprises a ring nut element which comprises a threaded hole configured to engage with said guide means, and said guide means comprises a first guide element configured as a threaded element, keyed onto said needle at a rear end thereof opposite to said free end, wherein said first guide element is configured to engage with said threaded hole of said ring nut, wherein said needle comprising said first and second guide elements (18, 20) is made of a single piece.