To get the shot/needle from Tinker Tom?

It really doesn't matter. I don't think it even does anything. Different companions might approve or disapprove of it though. You'll lose some health if you let him give it to you, but it's not a permanent loss. As soon as you sleep or heal you're back to your same full health.

i took it, nick loved that, and i lived to tell

lol I took the shot and it literally reduced me to 1 hp. Reloaded and said nope.