Anemia is usually the first sign of B12 deficiency,it's usually not advisable to supplement with 1 single source of VitB,Bcomplex is usually the best bet as all the B vitamins work together,in nature these vitamins do not occur as isolated entities but rather as part of the entire B vitamin complex,Bcomplex,especially B1 and B6, is often given to anorexic pets or too boost a poor appetite,B12 like other B vitamins is needed for metabolism of proteins,fats and carbs.It is also useful in stopping poop eating

I would be really interested to find out why you want to supplement with just B12 :)