[Candace]

If you ever lose the intrinsic factor that is needed for active absorption of B12, then you have to take 250 mcg to get the equivalent of the RDA via passive diffusion, or 500 mcg as insurance in case you're also in the 2-3% who needs more than the RDA. (The government figures that having 97-98% of the population well-nourished is good enough.) 1000 mcg is a little overkill, but it's cheap and harmless. No adverse effects have ever been reported for B12, so the government could not set an upper limit.