I had a similiar problem when my histamine levels became balanced. I continued taking my supplements as I had been and that caused me to have HIGH histamine levels (I had finished the loading phase, which caused my low histamine levels to become high). One problem was that my appetite was through the roof. I wanted to eat all the time. The solution for me was to lower my dose of niacin. That returned my appetite to what it had been before.

B12 also plays a role in methylation, and it relates to histamine levels. B12 can raise histamine levels. High histamine types are known for having a hearty appetite. (I was also eating too much sugary foods when my histamine levels were on the high side.)

I think you should talk to your doctor about the dose being too high. That would be the first choice. I think you may be done with the loading phase of the b12 supplementation, which means your tissues are saturated with b12 now. You should supplement b12 but at a lower dose now.

If you feel your appetite is not healthy, but basically out of your control, then it might indicate that the dose is too high. My appetite was not healthy but I had these cravings (for sugary foods) that I would only get when I had a yeast overgrowth. I know my appetite was not normal or healthy, but that is something you need to discuss with your doctor. (A yeast problem would cause an increase in appetite for ONLY sugary or starchy foods.)