Can You Take Too Much Vitamin B12?

VitaminB12Pills2 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.

5 thoughts on “Can You Take Too Much Vitamin B12?

  1. Bix Post author

    Everyone over 50 should be taking a B12 supplement (or eating B12 fortified food).

    It doesn’t matter what you eat, how much meat and dairy you eat, whether you’re a vegetarian or not. Food cannot correct a deficiency, and people over 50 are prone to deficiency because they secrete less stomach acid and intrinsic factor.

    Primary deficiency symptoms are anemia, inflammation along digestive tract (e.g. IBS), and nerve problems … numbness and tingling, poor balance, memory loss, trouble concentrating.

    As you know, I’m not big on supplements. But this one is an exception. If you take only one supplement, let it be B12.

  2. Melinda

    Thanks for this Bix. As someone with IBS, it’s very useful. Question: are there different types of B12 and one more effective or healthy than another? (I’m thinking here of vitamin D3 vs D2.)

    1. Bix Post author

      Both forms of B12 in supplements, cyanocobalamin and methlycobalamin, are absorbed well and can cure deficiency. The body converts cyanocobalamin into the more active methlycobalamin.

      I’ve read the pros and cons of these over the years and I’ve come to prefer the methyl- version.


      “No cyanide is released with methylcobalamin, although the amount of cyanide (2% of the weight, or 20 micrograms cyanide in a 1 mg cyanocobalamin tab) is far less than ingested in many natural foods.”

  3. Pingback: Vitamin B12: What Form Is Best? | Fanatic Cook

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