Following this question, I am wondering if there is a clear protocol/algorithm (I do not know the exact medical term) that a vegetarian/vegan can use to compute the recommended daily amount of B12 (maintenance doses).

I imagine that such an algorithm should take into account:

  • vegetarian degree (lacto, vegan etc.)
  • known B12 absorption issues
  • age
  • sex
  • B12 measured values (periodical blood analysis)

I find this particularly important, since most of the supplements (at least in my country) have relatively high amounts of B12 (starting from 500ug which theoretically is much more than the daily recommendation). This article illustrate more about possible side effects of high doses of B12 intake.

  • 500mg seems really high, are you sure if milligrams (mg) rather than micrograms (mcg/μg)? Is this oral (daily) supplements or injections?
    – Tom Kelly
    Commented Feb 4, 2017 at 13:28
  • @TomKelly - yes, it is clearly ug not mg. I have fixed the typo. I was talking about daily oral supplements. Thank you.
    – Alexei
    Commented Feb 4, 2017 at 13:30
  • Ah that makes more sense, still a bit high. 50μg is fairly standard (unless you are severely deficient) and should be available.
    – Tom Kelly
    Commented Feb 4, 2017 at 13:34

2 Answers 2


One basic thing to know is that B12 supplement absorption rate decrease with the total amount of B12 provided by the supplement (check here). In other words if you get your supplement daily then 1-2 mcg could be enough, but if you get it weekly it would be necessary to intake as much as 1000-2000 mcg.

The "standard" dose is 1000-2000 mcg per week, since is enough for most of people and it will not raise problems if your needs are exceeded by the supplement.

As for your request of a more precise protocol/algorithm, you will be delighted to read this exhaustive article: "B12 requirements, How Recommendations Were Formulated".

  • "B12 supplement absorption rate decrease with the total amount of B12 provided by the supplement" This is true for every macro and micronutrient, even if they are in real food.
    – Ramon Melo
    Commented Feb 5, 2017 at 18:01
  • 1
    @Attilio - yes, provided URL (and a little context) is exactly what I was looking for. Thank you!
    – Alexei
    Commented Feb 6, 2017 at 14:38
  • 1-2 mcg is not enough for a daily supplement. The range is more like 10-100.
    – Alex Hall
    Commented Nov 9, 2017 at 18:46

While vegetarianism is a consideration in testing for B12 deficiency (as you are at higher risk). B12 supplementation is usually advised based on monitoring levels in the blood rather than your demographics.

B12 is a stored vitamin so it may take a long time (months to years) to develop a deficiency. Therefore you are advised to take more than your body needs to restore these stores of B12 and monitor whether your B12 levels pick up with regular blood tests.

Another consideration is that B12 is poorly absorbed if taken orally. While the standard daily dose is 50μg (micrograms), only a fraction of that is absorbed. The daily intake varies (higher for pregnant or breastfeeding females) but is between 2-3μg for all adults and these supplements should be sufficient to absorb enough B12 if you have a marginal deficiency. While B12 overdose is rare, doctors advise against taking more than the standard 50μg dose as more than that is not necessary (mainly due to cost concerns rather than overdose). 50μg is the dose offered by several supplement companies (here in NZ) and higher doses for "energy" or "natural alternatives" are not recommended by doctors (as there is no evidence that they are beneficial).

Another option is injections of B12. Some people have issues absorbing this vitamin so it is recommended if B12 levels aren't restored by oral supplements or if patients have severe deficiency at diagnosis. Since it is a stored vitamin these injections may be given weeks or months apart (depends on your blood levels). This has more risks of overdose but is still widely regarded to be safe and is done routinely for patients with a variety of conditions (including Chronic Fatigue Syndrome). These injections are 1000μg (weekly) or 5000μg (3-4 weeks) at most. Note that while these are higher doses, the average daily intake is only marginally higher than oral supplements and these are intended to restore vitamin stores in deficient individuals rather than meet a daily need in those absorbing it regularly in the diet.

  • Since the supplement doses are standardised at 50, 1000, and 5000 μg, dosage is more based on whether it is oral or injections (and how often).
    – Tom Kelly
    Commented Feb 4, 2017 at 13:30
  • You mention that B12 overdose is rare. Can you provide any examples of B12 overdose?
    – Nic
    Commented May 23, 2018 at 17:20
  • No. It is so rare (virtually unheard of) that overdose is not a concern for medical practitioners. Higher doses are routinely given when required for various reasons. The main concerns for OTC treatments is cost and purity.
    – Tom Kelly
    Commented May 23, 2018 at 22:34

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