Finance and Business Does B12 make you lose or gain weight? Does B12 make you lose or gain weight? Despite the numerous processes in which vitamin B12 is involved, there's little evidence to suggest that it has any influence on weight gain or loss.Dec 16, 2019 Related Posts:What vitamins should not be taken with B12?Breakfast cereals that are high in vitamins b12 and b13 are fortified with them.How long does it take for a B12 shot to wear off?What to expect after getting a B12 shot?