It's not just that -- there's an active and a passive pathway for absorption. The active pathway is limited to 1-2 µg per day by the quantity of "intrinsic fact

It's not just that -- there's an active and a passive pathway for absorption. The active pathway is limited to 1-2 µg per day by the quantity of "intrinsic factor" produced by the stomach even for people with no absorption disorder, but about 1% of oral B-12 can be absorbed through passive diffusion. If you're trying to remediate a deficiency, intramsucular shots or high oral doses are the correct approach. I think there's another limitation in how much the liver can capture and process per day, and I don't know what that limit is, but it's higher. In any event, once it is processed (and after excess is eliminated), it is highly conserved in your body -- a few micrograms can last years, IIRC.