As a recently converted vegan, I first wanted to try out not taking B12 supplements to see if I can get the recommended RDA from fortified foods alone. This turned out to be rather hard and I average about 70% of the RDA per day.

I am interested, therefore, in taking B12 supplements to avoid defficiency, but I am unsure what concentration of B12 I should look for in a supplement. As I understand it, the RDA for B12 is somewhere around 2.5mcg/day, but the supplements I had a look at advertised anywhere from 25mcg to 1000mcg.

While I suppose there is some amount that doesn't get absorbed, 1000mcg still seems like a huge amount. Are supplements with such high concentration supposed to be taken on a day to day basis or is it okay to take them eg. once per week and let my body take care of it?

  • 1
    This was suspiciously easy, as if this question was set up knowing exactly what the answer would be.
    – Alex Hall
    Commented Feb 15, 2017 at 12:43
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    That would be an interesting approach to asking questions. :D I was just happy with your answer and felt that it answered my question very well and so I accepted it. If anyone else wants to answer I am of course interested in anything they have to say and may change accepted answer accordingly. Thanks for answering it. Commented Feb 15, 2017 at 12:47

1 Answer 1


From the Vegan Society:

To get the full benefit of a vegan diet, vegans should do one of the following:

  • Eat fortified foods two or three times a day to get at least three micrograms (mcg or µg) of B12 a day OR
  • Take one B12 supplement daily providing at least 10 micrograms OR
  • Take a weekly B12 supplement providing at least 2000 micrograms.

The less frequently you obtain B12 the more B12 you need to take, as B12 is best absorbed in small amounts. The recommendations above take full account of this.

From the vegan r.d.:

Current recommendations are for vegans to supplement with cyanocobalamin in one of the following three ways:

  • 2.5 mcg two times per day
  • 25-100 mcg daily
  • 1,000 mcg two times per week

As far as I know, this kind of thing always happens when supplementing any nutrient (though probably more for some nutrients than others). Concentrating a large amount of a chemical into a small pill makes it hard for the body to absorb it properly before it passes through. This effect gets worse as the dose increases, which means it gets even worse as it is compensated for. It's not impossible to overcome, but the result is that in order to get enough of the vitamin, you do need to ingest much more. So to get the RDA of about 3 µg in a single pill you need to take somewhere around 25 µg, and to get 7 days' worth in one go you need much more than 7 times that.

Note that the amounts above are for people who simply want to maintain a healthy level of B12. If you need to recover from a deficiency, you may indeed need to take something like 2000 µg daily:

For persons suffering from vitamin B12 deficiency and pernicious anemia: The recommended daily dosage is 300-10,000 mcg. There is also evidence suggesting that the most effective way of increasing vitamin B12 in such persons is through oral dosage of between 647-1032 mcg on a daily basis. (source)

The range here is so wide because it really depends on how deficient you are. If you suspect a deficiency, avoid looking for answers on the Internet - get a blood test and advice from a doctor!

  • So is this because of the nutrient being delivered as a pill? If the pill was crushed, or if the vitamin was sold as a solution, this would have an impact on the absorption? Commented Feb 15, 2017 at 12:49
  • @AlexanderRossa I think it would help, not sure how much. But at this point I'm really speculating.
    – Alex Hall
    Commented Feb 15, 2017 at 12:54
  • I see, thanks for answering that anyway :) Commented Feb 15, 2017 at 12:57

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