How is HIV transmitted through injecting drug use?

Sharing syringes is a high risk behavior for HIV since a contaminated syringe can pass blood directly from one person's bloodstream to another. Blood can enter a syringe when it is placed into the vein. Some of this blood remains in the syringe after use and can enter to bloodstream of others if the needle is shared. Sharing syringes is also a high risk for hepatitis transmission.