Assume I pull back and .. sure enuf I hit the pipleline and there is blood in the tube ...

Why must I waste a load and start from scratch ?

Couldn't I just yank the needle and move to a nearby injection site ? Why does my own blood contaminate the load ?

or is there some other issue at work?

All the Oncolgy sources say you dump the meds and start over. Unfortunately they do not give the details of why.