1: nothing
2: local infection
3: blood borne infection
4: you will develop superpowers
this is a really silly post, injecting other peoples blood will not get you high even if you hit the vein, regardless of what the donor has taken or how smashed they are at the time. If this is related to cocaine, then you will need to use actual cocaine.
If getting high wasnt your intention and you were doing a transfusion STOP now and get a medical professional to do it.
PS: please dont share any of your injection kit with anyone. Borrow someone elses if your really stuck, but keep it - they wont need to use it again