Vitamin B12 is important for creating red blood cells, which transport oxygen around the body. It also helps the body utilise Folic Acid, which not only helps create new red blood cells, but strengthens DNA so it's less likely to mutate and become at risk of cancer.
At least 1.5µg per day, that's a millionth of a gram. It's fine to take a more than this. See the table at the right for foods containing this amount.
Not really. Changes in urine color, heart rate and blood pressure are some small side effects of taking too much Vitamin B12. It's generally considered harmless for healthy people to overdose on B12.
It becomes harder to digest and absorb Vitamin B12 from foods as one gets older, so it is generally recommended to get Vitamin B12 from a supplement if you're over 50 years old.