He emphasizes with Ianto on that; it's... weird to see John touching Tosh, especially since that wouldn't have been possible just hours ago.
"Gratitude'll have to wait until we can be sure we should have it."
Once he finishes prepping the needle, he gets to work, taking care with it. He certainly couldn't be a full-time medic, but he can handle basic things like this. "Okay, true, still recovering from that Dalek invasion-- that'll give anyone plenty of reason to be untrusting, don't you think? Still... Torchwood still being around, and I don't know if I'll still be with them by then, if they went back to the old ways... Even if Earth isn't intentionally mucking about with it, there's still the risk of someone else having snuck something. There we go, Tosh. All done with that, and voila, needle out of sight."