Let me at the top indicate that medical science mostly says you  can't  really take too much Vitamin B-12.  For example, the Food and Nutrition Board has not established a Tolerable Upper Intake Level because it feels this is not necessary.  And sorry about this lengthy posting, but I thought you should see the entire  Quora  presentation. Keep in mind that the human body does not naturally produce its own vitamin B12: We obtain B12 from meat, fish, dairy, eggs and supplements. Bacteria in the large intestine can produce a small amount of B12, but the colon cannot absorb it. The recommended daily amount needed by the body is around 2.4 micrograms (mcg) per day. Note that those supplements you take are in the 500 mcg to 2500 mcg size tablets, which is taken daily is because your body is very inefficient in utilizing this vitamin.  What is not used passes through your digestive system. The older you get, B12 from the food you eat becomes insufficient, an...