Body stores of B12 are estimated to be about 7 mg. It's always been assumed that humans need around 1.5 - 2.5 µg per day, on the basis that this is the NET difference between the amount used up and lost, and the amount recycled by the body and reused. So on this basis, 1.5-2.5 µg per day will take 7 – 15 years to use up the whole of the body stores.

But the way the body uses B12 is by recycling. The body uses B12 for many things – energy production, nerve cell and cell membrane stability, growing cells, switching genes on and off, hormone production, antibody reactions, etc. The blood contains active B12 (Transcobalamin-II plus B12, called holotranscobalamin – with half life of a couple of hours), and inactive B12 (bound to something else or free – this B12 can't get into cells to be used). There are also considerable stores B12 in the liver. But inactive B12 can’t be used directly from the blood, and it is pushed into the gut by the liver (the bile duct), and then absorbed from the gut and combined with TC-II to make active B12.

So large quantities of B12 are pushed into the gut each day, and it all needs re-absorbing.

People with no B12 in their diet (vegans and vegetarians) still have normal recycling, so they can put B12 from the body stores (of inactive B12) into the gut and re-absorb it as active B12. According to the above calculations, it will take 7, 10, 15 or more years before the B12 stores in the body are depleted enough that signs and symptoms appear.

So vegans and vegetarians often say "these symptoms can't be because I'm a vegetarian, because I've been a vegetarian for so many years". It's like the oil level in a car – everything works fine as the oil level drops, over years of motoring. But when it finally gets below the oil pump intake, suddenly everything goes wrong at once.

Vegans and vegetarians can take oral B12 (tablets, lozenges, sprays) as a prophylactic, before the signs and symptoms appear. Once there are signs and symptoms, there is a likelihood that some of the absorption and transport systems no longer work, and typically vegetarians and vegans need injections too.

The opposite situation arises when people who don't absorb B12 (pernicious anaemia, and some other conditions) start showing signs and symptoms. In their case, large quantities of B12 from the body stores of inactive B12 are pushed into the gut, only they aren't recycled and they are lost. Exactly how much we don't know, but blood B12 can drop from 500 to 200 ng/L in four months!  If we assume original body stores of 5 mg, then this could mean losing 3 mg in 120 days, or approximately 30 µg per day!

Because the body can't absorb B12 from the gut, people typically need injections. By the time symptoms are recognised and diagnosis given, there may be damaged transport proteins and utilisation pathways, these people may need to maintain a very high blood serum concentration of B12 in order to function.

Everyone needs different levels of blood serum B12 to function properly, and everyone needs a different frequencies of injections to maintain their blood levels. This means that some people are fine getting their injections every three months, some people need them every two months or monthly, and some people actually need even more frequent injections. Some people need intravenous infusion, for quantities of 10 mg at a time or more – but luckily, these are very rare. Typically, injections of 1 mg (1000 µg) are quite sufficient.