I take them because I am fatigued and low on B12 thanks to my Lyme Disease. It does ease the fatigue some, but I have never heard of taking them for weight loss. You need a prescription to get them and they cost about $125 a month from a compounding pharmacy.
B12 does not aid in weight loss. It increases energy. You can get it at Wal-mart now sub-lingual. Add few drops under the tongue. The increase in energy leads you to be more active helping aid in weight loss, but B12 alone does not make you lose weight.