Vitamin B12 is also called as Cyanocobalamin. Methylcobalamin and 5 deoxyadenosylcobalamin are the forms of vitamin B12 that are active in human metabolism.

Functions

Vitamin B12 is required for proper red blood cell formation and DNA synthesis. Vitamin B12 is also needed for building proteins in the body and normal function of nervous tissue
Rich Foods
1) Beef liver 3 oz: 18 mcg (over 100% DV) 
2) Sardines 3 oz: 7.6 mcg (over 100% DV)
3) Beef (grass-fed) 3 oz: 1.5 mcg (25% DV)
4) Tuna 3 oz: 2.5 mcg (41% DV) 
5) Raw cheese 1.5 oz: 1.5 mcg (25% DV) 
6) Cottage cheese 1 cup: 1.4 mcg (23% DV) 
7) Lamb 3 oz: 2.07 mcg (35% DV)
8) Raw Milk 1 cup: 1.1 mcg (18% DV) 
9) Eggs 1 large: 0.44 mcg (7% DV) 10) Salmon 3 oz: 1.1 mcg (18% DV)etc..

Overdose

Numbness, improper heart functioning, giddiness and regular headaches. The most serious side effect linked to overtime abuse of this vitamin, is increasing the risk of getting cancer.If you are undergoing pernicious anemia, an overdose of this vitamin can lead to leukemia. 

If you are consuming a diet that has a high concentration of cholesterol along with different animal proteins, you stand a very high chance of getting cancer of the esophagus and stomach.

Deficiency

B12
Deficiency of vitamin B12 leads to strange sensations, numbness or tingling in the hands, legs or feet, difficulty walking (staggering, balance problems), anemia, a swollen, inflamed tongue,yellowed skin (jaundice).