ok heres what you do, look around outside and find a pile of dog shit. Dog shit has cleansing properties due to the nature of their immune system.
Rub it on the area you injected. Then let it sit for awhile prob 10 minutes.
After that jump in the shower and wash it off. While you are in there punch your self in the nuts four times.
This should fix your problem.. or at the least remind you next time that its not smart to inject yourself with a dirty needle..
Ps.. the dogshit thing isnt real so dont try it..