A doctor who injected a willing patient that came to them, or a recognised guy from the local hospital helping an unconscious victim? Yes usually, people won't turn hostile.
But if a random guy I just met in the street just went and injected one of my friends out of the blue, without consent and without explaining what they were about to do? Yup, I'm going to turn hostile, even (or maybe even especially) if my friends behavior suddenly becomes wierd. Maybe not "roll for initiative" hostile, but putting myself between them, angrily asking for an explanation, and calling the police, certainly. And if the guy continues to try and interact with my now-dosed friend, without giving me a satisfactory explanation, then we'll roll init' and use the graple rules.
And I'm mostly a peaceful guy, bordering on cowardly. But there is a big difference between a medic doing their job, and a random dude doing weird **** to someone I care for without asking for their consent.