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 (L) 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 (L), 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 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 (L) 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 (L), 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 is made of a single piece. <EOS>