I believe you are absolutely right about consent. With another adult, it can be tricky. Recently I was leading a guided trip on San Miguel Island, and one of the group said to her comapnion, "Please give me an ibuprofen when we get back to the boat." I overheard this and mentioned that I had some ibuprofen with me. She asked if I would provide one for her, and I did. She took the initiative - I jst happened to have what she desired.
It would have been different if I had walked up to her and said "You look like someone who could use an obuprofen. It so happens I have some right here and I can save the day."
In general, my FAK is light on meds - only aspirin and "vitamin I."