This is taken from a response to a question posted on Quora, found here, and I thought it might be valuable to have it posted here.
Generally speaking, water soluble vitamins are easily absorbed into the body, but any excess is excreted, not stored. They include all of the b-complex vitamins and vitamin C. They need to be consumed regularly in order to support normal body function.
Fat-soluble vitamins include A, D, E, and K, and are more readily retained by the body (in the liver). They are then released into the bloodstream and do not need to be consumed on a daily basis to remain healthy. However, it is possible to consume them in excess, which can lead to its own set of issues (but that’s a topic for another post).
Vitamin B12 is necessary for normal functioning of the brain and nervous system, as well as the formation of red blood cells, and actually is stored by your body - usually between 2 and 5 mg in adults. Roughly 50% is stored in the liver and the majority of the remainder is found in your muscles.
B12 deficiency can lead to a disease called pernicious anemia, which occurs when your body doesn’t have enough red blood cells. Initial symptoms include fatigue, shortness of breath, pale skin, chest pain, among others. Without going into too much detail about the condition, just know that without treatment it can lead to permanent neurological damage and eventually death.
It is theorized that B12 deficiency is more widespread in the developing world because of low intakes of animal products, particularly among the poor.
For those individuals requiring supplementation, 1–2 milligrams is usually sufficient. Roughly 1–5% of high oral doses of crystalline B12 is absorbed along the entire small intestine by passive diffusion.
The 1500 mcg tablet the original author has access to is consistent with supplementation for individuals that have a demonstrated deficiency.
For healthy adults, daily consumption of 2.4 mcg is recommended, as your body typically will only lose roughly 1.5 mcg/day.
I hope this clarifies things somewhat.