Not, vitamin B12 not fattening.
Vitamin B12 is important in the manufacture of new cells, such as red blood cells, in addition to being essential for the proper functioning of the nervous system.
In pregnant women and women of childbearing age who intend to become pregnant, vitamin B12 is very important to development appropriate nervous system of the fetus, avoiding some malformations. Therefore, it is common to use folic acid at least 3 months before conception and during the first 3 months of gestation.
Usually, anyone on treatment or supplementation takes the pill every day and in no way does it gain weight or increase appetite.
If you are taking vitamin B12 and is getting fat, it is probably for other reasons which need to be investigated.
Seek medical advice to properly guide you about using vitamin B12.