The Institute of Medicine (IOM), the group that develops the RDAs, says that people over 50 should take a B12 supplement, they shouldn’t rely on food. Can you take too much? They didn’t set an upper limit:
There is not sufficient scientific evidence to set a Tolerable Upper Intake Level (UL) for vitamin B12 at this time.
As with many nutrients, the more we consume, the less we absorb. In the case of vitamin B12, we absorb just 5% of a 25 microgram dose:
Nearly 50% was retained at a 1-µg dose, 20% at a 5-µg dose, and just over 5% at a 25-µg dose.
The RDA for vitamin B12 for people 19 to 70 years old, men or women, is 2.4 micrograms/day. There is a proposal to raise this to 4-to-7 micrograms/day.
Here’s what the IOM says about taking a supplement containing more than the RDA:
No adverse effects have been associated with excess B12 intake from food or supplements in healthy individuals.
When high doses are given orally only a small percentage of B12 can be absorbed from the gastrointestinal tract, which may explain the apparent low toxicity.
On the basis of the review of data involving high-dose intakes of B12, there appear to be essentially no risks of adverse effects to the general population even at the current ninety-fifth percentile of intake noted above. [37 µg/day]
There appear to be no risks associated with intakes of supplemental B12 that are more than two orders of magnitude higher than the ninety-fifth percentile of intake. [2 orders of magnitude is x times 100, or 37 times 100, or 3,700 micrograms, or 3.7 mg]
When we consume more than a couple micrograms at any one meal, most of the excess goes out in the feces. However:
When large doses of crystalline B12 are ingested, up to approximately 1 percent of the dose may be absorbed by mass action even in the absence of intrinsic factor.
So, if you take, say, a 500 microgram tablet, 1 or 2 micrograms will be absorbed actively, and maybe another 5 micrograms will be absorbed passively. If that ends up being more than blood can carry:
If the circulating B12 exceeds the B12 binding capacity of the blood, the excess is excreted in the urine.
The answer to the question in my title? It would be difficult to take too much vitamin B12 … 3,700 micrograms/day appear to show no risk. And the body has a number of ways to get rid of a large dose.