Medical question

About three years ago my doctor prescribed me a bottle of potassium, to help my kidney. When that bottle ran out, I took the initiative to go out and get more, over-the-counter. (Apparently this was unnecessary. She only wanted me to take that one bottle.)

But my question is, I Just went back to the doctor today. She said that OTC potassium is useless.(?!) She said there is a difference. And the only good potassium is prescription.

That makes no sense to me. I would think potassium is potassium. If it's #19 on the periodic table of elements, it's potassium. What difference would a prescription make?

Has anybody else ever heard this? Because it's new to me.