sometimes i like to think about things like items etc i'd add to a version of my setting where it's specifically some sort of video game. here's one that just popped into my head right now:
Nanite Revival Canister
A small canister of nanites. These ones are programmed to detect when the host is dying and rapidly repair any severe damage or injury. The process expends their entire energy reserve and severely deteriorates them, making the effect single-use. Experts recommend not having more than one dose of these in your bloodstream at a time, as any more could interfere with your immune response and/or any other nanite-based medications or auto-doctor systems present.
you would be able to take more than one at a time, but having more than one active would, in fact, fuck with your healing over time auto-docs if you have them. and would also make any other nanite stuff you try to give yourself more likely to fuck up or only work at a reduced capacity. and make you more likely to get sick
the effect of using up a dose would, of course, be returning you to ~50% health upon taking an otherwise lethal blow. probably repair any damaged body parts up to a minimum threshold of "usable but still fucking hurts" as well
as for where the used-up nanites go, well. same place your red blood cells go when they die.