Formal or Informal

I am a nurse studying spanish in order to communicate more elegantly with my patients. What is the proper way to address my patients? With the formal because they are strangers to me, or with the informal because I am a "person in an authority" position? :roll:

As a healthcare provider myself, I always address patients formally first, then ask their preference. Regardless of authority, respect in the this instance, is paramount. JMHO. buen dia.