It cannot be called C12 even if the central molecule is cobalmin. It is called B12 only.
Research is suggestive of no interactions between vitamin B 12 and vitamin D.
When you do not get enough of vitamin B12 then your body feels lethargic and fatigued. Additionally, you may also suffer from poor digestion.
Yes, a B12 deficiency can cause stomach upset and poor digestion.
The role of vitamin B 12 is very crucial as without enough red blood cells, your tissues and organs don't get enough oxygen. As a result your body cannot work well. You will suffer from weak musles, numbness and extreme fatigue.