Vitamin B12 Overdose

Vitamin B12 is one of the water-soluble vitamins. Excess is therefore eliminated with the urine. Nevertheless, it has recently been said again and again that vitamin B12 can also overdose. We clarify whether this is the case and how such an overdose of vitamin B12 should manifest itself.

Why an overdose of vitamin B12 is hardly possible

Many vitamin B12 preparations are commercially available in high doses, so there is often a fear of an overdose of vitamin B12. The official daily requirement for vitamin B12 is between 2.4 and 6 µg (depending on the institution/source ( 18 )), but many vitamin B12 preparations contain 150 µg, 500 µg, or even 1000 µg, which of course is a multiple of the alleged need. Nevertheless, an overdose of vitamin B12 is hardly possible, especially through oral intake.

Vitamin B12: No evidence of toxicity or overdose

On the website of Harvard Medical School (the medical faculty of the renowned Harvard University), one reads that the Recommended Dietary Allowance (RDA) for vitamin B12 – i.e. the amount of the nutrient that is sufficient to cover the daily requirement – for adults is 2, 4 µg lie. If possible, however, one should take 6 µg.

However, there is no official limit as there is no evidence of toxicity or the possibility of an overdose when it comes to vitamin B12. Only people with what is known as Leber’s optic atrophy – a rare hereditary eye disease – should avoid taking vitamin B12 supplements.

Poisoning or overdose symptoms due to vitamin B12 are therefore not known. Because vitamin B12 is a water-soluble vitamin, the excess is broken down by the kidneys and eliminated with urine.

Vitamin B12 deficiency is more of a danger than overdose

Many people also suffer from a vitamin B12 deficiency because the vitamin can only be absorbed in very small amounts, which further reduces the risk of an overdose.

A special transporter protein is required for the absorption of vitamin B12 into the bloodstream. It’s called an intrinsic factor and is produced in the stomach. Too little intrinsic factor is often formed in the case of chronic stomach problems, when taking certain medications, or in general in older people (65+). Consequently, too little vitamin B12 can now be absorbed. There is a vitamin B12 absorption disorder, e.g. when taking acid blockers or metformin.

Only small amounts of vitamin B12 can be absorbed per intake

But even with sufficient intrinsic factor formation, only 1.5 µg of vitamin B12 can be absorbed via this so-called active absorption per meal or per dietary supplement intake, as we already explained in our article on diagnosing and remedying a vitamin B12 deficiency to have.

So even if a dietary supplement contains 1000 µg of vitamin B12, only 1.5 µg of vitamin B12 can be absorbed through active absorption. An overdose is therefore not possible at all since the body acts in a self-limiting manner here. But why are dietary supplements with vitamin B12 offered in such high doses at all? Quite simply: because there is passive resorption in addition to active resorption.

Overdose is not possible even through passive absorption

In the case of passive absorption, vitamin B12 can also be absorbed without transporter proteins by means of diffusion through the intestinal mucosa and is therefore ideal for people with a vitamin B12 absorption disorder.

However, the amounts of vitamin B12 absorbed in this way are only 1 percent of the vitamin B12 consumed/ingested. If you take the usually recommended 2.4 to 6 µg of vitamin B12, you will not notice passive absorption, since 1 percent of 6 µg makes up just 0.06 µg and is therefore negligible.

If you take a high-dose vitamin B12 supplement instead, e.g. B. 500 µg, then thanks to passive absorption, a total of 6.5 µg can get into the blood and thus wonderfully cover the daily requirement. The 6.5 µg are made up of the 1.5 µg from active absorption and the 5 µg from passive absorption. Even in the event of an absorption disorder – when active absorption no longer works – 5 µg of vitamin B12 still gets into the blood, which covers the requirement but is still a long way from an overdose.

Vitamin B12 overdose through vitamin B12 injections?

With vitamin B12 injections, you bypass the limited absorption capacity in the digestive tract and the vitamin quickly gets into the blood. Nevertheless, the risk of an overdose is no higher here than with oral intake – as two studies have shown since even after several vitamin B12 injections the vitamin B12 level did not rise noticeably higher than when the vitamin was taken orally.

In one (from 2003) the subjects – all of whom were vitamin B12 deficient – ​​received 1000 µg of vitamin B12 daily for 10 days, then once a week for 4 weeks, and then once a month continuously. One group received this dose orally, the other intramuscularly, i.e. as an injection. Vitamin B12 levels recovered equally well in both groups.

In the other study (from 1998), one group of subjects – who were also vitamin B12 deficient – received 1000mcg B12 injections nine times over the course of four months, and the other group took 2000mcg daily by mouth. In the end, vitamin B12 levels recovered even better in the oral group.

However, there are also case reports showing that injections can lead to excessively high vitamin B12 blood levels. In a collection of five case reports, two of these reported that B12 shots increased B12 levels to 2840 pg/mL and 1839 pg/mL, respectively. Values ​​of 200 to 900 pg/ml are considered normal. Injections can therefore be used very well as a cure to remedy a deficiency, but oral intake is more suitable for a regular supply of vitamin B12.

Acne as a symptom of vitamin B12 overdose?

After taking B vitamins, especially B12 supplements, some people develop impure skin with pimples, a type of acne. However, according to individual case reports, this can also occur with low doses of vitamin B12, e.g. If, for example, only 20 µg occurs, this is not a sign of an overdose, but rather of an individual intolerance (possibly genetic). In the corresponding reports, not only vitamin B12 was often taken, but also other B vitamins that are also possible acne triggers (e.g. vitamin B6).

Another reason for acne after taking vitamin B12 could be a simultaneous biotin deficiency. Schweikart explained. Especially high vitamin B12 starting doses could use up the biotin stores because biotin is a cofactor of vitamin B12, which means that both substances need each other for optimal effectiveness. If you develop impure skin after taking vitamin B12, try whether taking biotin (e.g. 100 to 300 µg per day) brings improvement here.

So if you have a vitamin B12 deficiency, whether due to a plant-based diet or an absorption disorder, you can remedy it more quickly if you take high-dose vitamin B12 supplements between 500 and 1000 µg. According to one study, especially in the case of an absorption disorder, the vitamin B12 level only increases from doses of 500 µg. If you stick to the official minimum quantities in this case, you will never get out of your deficiency and will continue to suffer from the corresponding deficiency symptoms.

Vitamin B12 overdose during pregnancy

Even during pregnancy, there is no need to worry about an overdose of vitamin B12 if the mother-to-be takes 500 to 1000 µg of vitamin B12 if she finds that she has a deficiency or needs more. On the contrary, a good supply of vitamin B12 is extremely important for brain and nerve development in the embryo, while a vitamin B12 deficiency can increase the risk of neural tube defects, premature birth, preeclampsia, and miscarriage.

Officially, there is a warning against overdosing on vitamin B12

Nevertheless, lately (2020) there have been repeated warnings about overdoses of vitamin B12. For example, on April 28, 2020, the NDR program Visite published an episode entitled “Dietary supplements: Be careful with B vitamins”. It said that many vitamin B12 supplements overdosed. They contained significantly higher amounts than the daily requirement. There is a risk, but no benefit.

The advantage lies – as you now know, in contrast to the NDR editorial team – in passive absorption, which is the key to remedying a vitamin B12 deficiency, especially if there is an absorption disorder.

High doses of vitamin B12 are risky because they increase the risk of lung cancer and death, the NDR program explains. We examined the subject in detail in our article on the allegedly increased risk of lung cancer from B vitamins and came to the following conclusions:

  • Only a link was found between lung cancer and high vitamin B12 levels, without examining what caused the vitamin B12 levels to increase and without examining whether the people in question had taken vitamin B12 at all. Because a vitamin B12 level can rise excessively for completely different reasons, e.g. B. if chronic diseases are already present.
  • If there was a link to B vitamin supplementation, then the problem was either a folic acid overdose or that the subjects were smokers. In the latter case, however, finding the culprit in the vitamins is paradoxical.


  • Studies that show that long-term intake of high-dose B vitamins does not result in an increased risk of cancer and are also available are not mentioned.


  • Prof Dr. Martin Smollich (specialist pharmacist), who spoke on the NDR program, cites a study from 2020 as evidence of an increased risk of death from vitamin B12, which showed that the higher the vitamin B12 plasma level, the higher the risk of death. But here, too, it was only about increased vitamin B12 levels, the cause of which nobody knows. Interestingly enough, those people who took vitamin B12 as a dietary supplement were NOT allowed to take part in the study.


  • Three other studies that Smollich cited are exclusively cell experiments, which showed that vitamin B12 is used not only by healthy cells but also by cancer cells and can promote their spread and proliferation. However, cell experiments are generally not considered representative, since one cannot conclude from them that the same effects occur in the living organism. Apart from that, cancer cells use almost every substance that healthy cells also use. Of course, excessively high B12 blood levels should be avoided. But that applies to every substance!
  • This vitamin B12 blood level is considered too high
    Vitamin B12 serum levels of over 900 pg/ml or over 665 pmol/l are considered excessively high.

The actual causes of too high a vitamin B12 level

If Professor Smollich and colleagues are right and an increased B12 serum level indicates an increased risk of cancer and death, but the B12 level rarely increases to such an extent when taking vitamin B12, the causes of an excessive B12 serum level should urgently be clarified will. Because an increasing vitamin B12 level is more likely to be the result of an existing or developing disease, e.g. B. rheumatism, liver diseases (hepatitis, liver cirrhosis, liver cancer), kidney diseases, blood diseases (e.g. leukemia) or cancer (breast, colon, stomach, pancreas cancer).

Note that the serum B12 level is not suitable for revealing a vitamin B12 deficiency.

Avoid overdosing on vitamin B12

As explained above, by taking vitamin B12 in doses of up to 1000 µg per day, it is hardly possible to achieve rising vitamin B12 blood levels beyond a healthy level.

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