Does this mean "no consent given for the medication" or "medication that is not part of an agreed consensus"?
If the former, then there are literally tons of occurrences of this everywhere.
I am also not sure of the implications that it entails. If I find someone injured on the street, and he tells me he does not want help, would it then be against the law for me to discard his instructions and help him?
Are there specific types of situations you're thinking about? Outside of cases where the patient is unable to provide consent (minority, mental incapacity, unconscious in exigent circumstances), I believe this is largely followed in the US, at least. Childhood vaccines, for example, are optional if you don't want to attend public (or many private) schools.