"Oh, they're dangerous," John assures him. Comfortingly. "Can cause numbness, vomiting, paralysis. That's why I'm stunning them first!"

Which makes it considerably safer but perhaps... but entirely safe, in truth. Still, small details! Stunning them will help and they'll be careful. It'll be fine.

"Lysa gave me something we can take if we need it, but I'm sure we'll be fine. Maybe you can ask her how it all works when we hand them in."

See? That's how you get your answers, Ianto! By finishing the mission first. John unholsters his gun and begins to check it over one-handed, making sure the cartridge is charged. His limited movement is, perhaps, not that reassuring.