vitamin B12 Any member of a group of cobalamin vitamers that exhibit biological activity against vitamin B<small>₁₂</small> deficiency. Vitamin B<small>₁₂</small> deficiency is associated with low red blood cell count and anemia. The vitamers are found in foods such as cereals, meat, fish, and poultry. The vitamers include adenosylcobalamin, hydroxocobalamin, cyanocobalamin, aquacobalamin, nitritocobalamin and methylcobabalamin (also includes their ionized, salt and phosphate derivatives). B vitamin