This phrase is never meant to be taken literally.
No-one really injects coffee directly into their blood stream.
Caffeine is the main ingredient of coffee, and gives us a feeling of
satisfaction. It is simply a joking expression which refers to an intense love, or even addiction, to coffee.
I'm
going nuts - If I
don't shoot caffeine intravenously soon I'm going to fall out of my tree!
When I was 13 I had my first coffee, and I've been practically
shooting caffeine intravenously ever since!