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!
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