An injection device comprising: a housing; a carrier disposed within the housing for holding a syringe that has a needle at one end; a needle sleeve located within the housing and being axially moveable with respect to the housing between an extended position and a retracted position; a biasing element configured to bias the needle sleeve towards the extended position; the injection device comprising a locking mechanism configured to lock the needle sleeve in the extended position after use of the injection device, the locking mechanism comprising; a first locking member provided on and projecting from the carrier; and a second locking member comprising a deflectable arm provided on the needle sleeve and moveable between a relaxed position and a deflected position; wherein the housing includes an inwardly projecting boss which engages the second locking member and moves the second locking member into the deflected position as the needle sleeve is moved to the retracted position; the first and second locking members (26, 30) configured such that the first locking member is out of engagement with the second locking member when the second locking member is in the deflected position; and the first and second locking members (26, 30) configured such that the second locking member blocks the first locking member when the second locking member is in the relaxed position and the needle sleeve is in the extended position, thereby preventing movement of the needle sleeve back into the retracted position. a housing; a carrier disposed within the housing for holding a syringe that has a needle at one end; a needle sleeve located within the housing and being axially moveable with respect to the housing between an extended position and a retracted position; a biasing element configured to bias the needle sleeve towards the extended position; the injection device comprising a locking mechanism configured to lock the needle sleeve in the extended position after use of the injection device, the locking mechanism comprising; a first locking member provided on and projecting from the carrier; and a second locking member comprising a deflectable arm provided on the needle sleeve and moveable between a relaxed position and a deflected position; a first locking member provided on and projecting from the carrier; and a second locking member comprising a deflectable arm provided on the needle sleeve and moveable between a relaxed position and a deflected position; wherein the housing includes an inwardly projecting boss which engages the second locking member and moves the second locking member into the deflected position as the needle sleeve is moved to the retracted position; the first and second locking members (26, 30) configured such that the first locking member is out of engagement with the second locking member when the second locking member is in the deflected position; and the first and second locking members (26, 30) configured such that the second locking member blocks the first locking member when the second locking member is in the relaxed position and the needle sleeve is in the extended position, thereby preventing movement of the needle sleeve back into the retracted position.