I currently take a supplement that contains both berberine and Ceylon cinnamon. Back when I took berberine and cinnamon separately, I mixed a bit of the Ceylon cinnamon in my breakfast drink. One can also take it in capsule form.
Perhaps your doctor is afraid that taking both Metformin and berberine might make your counts go too low.

I did it two ways - capsules before I went to bed. (But look out for cinnamon burps, they are painful!) And putting it in my oatmeal in the morning. The capsules worked best for me. Metformin doesn't seem to work for everyone. Are you on glipizide? I'm not familiar with Berberine. I am on Mounjaro and don't seem to have any of the side effects that folks on Ozempic deal with - you might check into that too.