[Candace]

1000 mcg of B12 isn't as large as it sounds. Doses that large are ~0.5% absorbed, so you only absorb 5 mcg. RDA-sized doses (2.4 mcg) are absorbed 50% so you'd absorb 1.2 mcg from them. Therefore, a 1000 mcg dose is only effectively 4 x RDA.