For causation to exist then generally speaking it would have to be reasonably foreseeable e.g. you cut someones finger, it gets a slight infection and they are given antibiotics, then infection and wound heal - normal chain of events that could be expected compared with you cutting someones finger, a doctor deciding to give them chemotherapy and amputate a leg - such a course of action is not reasonably within the boundaries of what you might expect from the initial action. In the latter scenario (unlikely as it is) the chain of causation would be interrupted and your initial action in cutting the persons finger has your guilt (and therefore liability) halted as the point of potential remedies for the cut finger rather than anything resulting from the amputation or chemotherapy.
As also mentioned the thin/eggshell skull rule probably applies. Simple solution of course is not to assault anyone and you don't run the risk of unexpected (for you) repercussions if/when they don't accept medical treatment.
Intent may well play a part in it and mitigate any guilt on your part but it's difficult to give anything more than a very general overview without specifics.