B-12 is destroyed in the stomach by acids if the parietal cells in the stomach do not produce enough intrinsic factor.
This is why many elderly get B-12 shots to bypass the stomach.
I’ve read that Vitamin C and B12 also do not get along and should not be taken together, but 3 hours apart. I’ve also seen that Calcium, Magnesium and Zinc should not be taken together yet I see supplements all over stores that contain just these three vitamins together. Drive a man nuts trying to figure this all out.