Why are we precsribed 1000 mcg/day of b12 for deficiency? and why not higher dose, e.g. 2000/3000/5000 mcg?

More not better: Your body has a threshold at which adding more does not result in a better result, rather it is simply urinated out of the system. B12 is a water soluble vitamin. Adding more beyond the saturation point does not make things necessarily happen faster.

Urinary Excretion: The bone marrow can only use a certain amount of B12 at a time to produce red cells. The remainder will be excreted in the urine. It would be useless to take more. Thanks for trusting HealthTap!