RDA is only 1 microgram.
Your liver stores vitamin B12, and under normal circumstances, the store will last for many years.
Where do you get vitamin B12?
You get it from liver, oily fish, dairy products and eggs.
You need vitamin B12 to keep a healthy nervous system and to make red blood cells. It also stimulates the appetite, encourages growth and releases energy from food. It helps you fight infections, speeds thought and protects against allergies and cancer.
Too much vitamin B12
Toxicity is rare even in high doses, as little is absorbed from the gut.
Too little vitamin B12
Deficiency of the so-called "intrinsic factor" in the gut prevents it being absorbed from food. Over time, the lack of B12 results in pernicious anaemia, which can be fatal if not treated.
Symptoms of pernicious anaemia include muscle weakness, mental slowness, confusion, poor co-ordination, shortness of breath, fatigue and numbness in the limbs.
Doctors treat pernicious anaemia by vitamin B12 injections for the rest of the patient's life.
If you eat a strict vegan diet, you may need to supplement with vitamin B12.
Vitamin B12 supplements
Supplements are most effective if other B vitamins, vitamin C and minerals are taken at the same time.