Hey, Rhi! Thanks for your suggestions!
Needles, though messy, are the fastest ways to administer many things. My fic is set in 2007, and I do have syringes being used in it. So needles won't be a surprise for my readers, though I'd really like a painless alternative.
This doesn't seem plausible, because Voldemort could have merely magicked some deadly poison into Harry's veins and killed him, then.I don't really have any suggestions, apart from charming nutritive potions into their bloodstream/digestive system, depending on the level of accident.
And my character has been stabbed and the poison in the knife has caused them to slip into a coma.
Of course, CT/MRI scans will be there, and I do have wizarding alternatives for them. And for BP, temperature and pulse check, I've got suitable stuff. It's just the drip that's bothering me.