What are the side effects of vitamin B12?

Was können Nebenwirkungen von Vitamin B12 sein?

Vitamin B12 is an all-rounder among vitamins: It plays a major role in our metabolism and has important tasks in the nervous system, blood formation and the breakdown of fatty acids.

Vitamin B12 is essential for our nervous system and its processes. If there is a lack of vitamin B12, side effects on nerves, muscles or the eyes can occur. You must not underestimate this in the long run.

Our article aims to enlighten you about the essential functions of this vitamin, as well as side effects of an underdose or overdose. You will also receive good tips and tricks on how to take vitamin B12 successfully and how to take precautionary measures for any overdoses or underdoses.

the essentials in brief

  • Vitamin B12 is responsible for the formation of red blood cells, tasks in the nervous system and the breakdown of fatty acids.
  • Vitamin B12 is taken in through food and can be stored in the liver for years.
  • There are hardly any side effects from an overdose of vitamin B12. A lack of vitamin B12 can cause problems in the nerves, muscles or eyes.

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What is vitamin B12?

Vitamin B12 are actually cobalamins, so-called chemical compounds. These cobalamins are called the vitamin B12 group. In this B12 group is the coenzyme B12, which is part of several enzymes and gives vitamin B12 its name. These enzymes take part in the metabolism of amino acids. (1)

Put simply, vitamin B12 is very important for cell division, blood formation and the functioning of the nervous system. In the human organism, vitamin B12 is mainly absorbed via the liver and kidneys. The human large intestine is able to produce vitamin B12 in very small amounts.

Vitamin B12 is produced naturally by microorganisms. These microorganisms are bacteria.

People absorb vitamin B12 primarily through their consumption of meat or offal. Therefore, vegans in particular have a greater risk of vitamin B12 deficiency. For comparison, here is a table of foods. The cobalamin content is compared. However, the specified cobalamin content does not correspond to the vitamin B12 content that can be used by the human organism.

Groceries Content in micrograms / 100g
beef liver 65 µg/100g
pork liver 40µg/100g
Salmon 2.9 µg/100g
Emmental cheese 2.2µg/100g
soy sauce 0.3µg/100g
Vegetables 0.01µg/100g

As you can see, there is a small amount of vitamin B12 in many foods that come from animals. Unfortunately, plants do not yet have a sufficient proportion of vitamin B12 for humans. Nevertheless, research is constantly being carried out into other vitamin B12 sources from non-animal origin.

How does vitamin B12 work in the body?

Vitamin B12 is a water-soluble vitamin that is stored in our body for years. It is mainly stored in the liver, usually for several years. Vitamin B12 is important for the formation of red blood cells and the nervous system.

People with stomach or intestinal inflammation have difficulty absorbing vitamin B12.

In addition to the formation of red blood cells, vitamin B12 is also important for the development of nerve cells in the spinal cord and many reactions in the protein and nucleic acid metabolism.

Vitamin B12, also known as cobalamin, is introduced into our body through the mucous membrane cells in the intestine. The intrinsic factor , which is a special protein, is important for the absorption of vitamin B12. This special protein is produced in the stomach lining and enters the intestines with the food. (2)

When is vitamin B12 dangerous in the body?

The dangerous thing about a vitamin B12 deficiency is that a deficiency does not appear overnight, but rather comes on gradually. Possible symptoms that may appear are tiredness, a pale complexion, difficulty concentrating and memory problems. Tingling in your hands or feet can also indicate that you have a deficiency.

different colored tablets in a glass cup

An overdose of vitamin B12 through ingestion of food is impossible. (Image source: Adam Niescoruk / unsplash)

Our body can store large reserves of cobalamin. Therefore, many years can pass before a vitamin B12 deficiency is even noticeable in the blood count. And this is what makes it so dangerous: The deficiency can cause irreparable damage to the nervous system if the lack of cobalamin is not discovered for a long time. In older people, memory loss or dementia are often associated with vitamin B12 deficiency. (3)

That's why seniors, type 2 diabetics and people who suffer from stomach or intestinal problems should have themselves checked regularly. The vitamin B12 can only be properly utilized if the stomach and intestinal tract are intact.

How do I know that I have a vitamin B12 overdose?

An overdose of vitamin B12, cobalamin, is generally only possible when injected through a syringe. You cannot overdose on vitamin B12 through food. Amounts of vitamin B12 that are not required are ignored by the intestinal wall and excreted again via the kidneys.

An overdose is possible due to:

  • Hepatitis (chronic and acute)
  • high intake of vitamin B12 (e.g. through injections)
  • leukemia

If the vitamin B12 supply is injected through injections or infusions, symptoms of intolerance can also occur here. These symptoms are:

  • eczema
  • local allergic reactions
  • acne
  • allergic shock

An overdose of cobalamin is hardly possible and is even recommended in therapy. Oral preparations in tablet form are just as effective as injections. Toxic overdose is not possible because the body excretes too much vitamin B12 by itself.

What are the side effects of a vitamin B12 deficiency?

Vitamin B12 deficiency occurs when less vitamin B12 is taken in than the body uses over a long period of time or over several years. In addition to normal consumption, medication can also promote vitamin B12 deficiency.

The following triggers for a vitamin B12 deficiency are:

  • Lack of intrinsic factor (the protein produced in the stomach and needed for absorption)
  • Impaired absorption of vitamin B12 in the intestine
  • Increased consumption of vitamin B12 and insufficient intake

Our body needs enough vitamin B12 for healthy blood formation, nerve function and cell division. If there is a deficiency, symptoms can occur. These affect, among other things, the muscles, nerves and hair, as well as eyes.

The following symptoms can occur:

  • Degeneration of the optic nerve
  • weak muscles
  • hair loss
  • Disorders of cell division and depth sensitivity
  • depression, fatigue

It is important to detect vitamin B12 deficiency early enough. Since neurological symptoms such as sensory disturbances can be irreversible, you should see a doctor early enough. A deficiency can be treated well with vitamin B12 therapy.

What role does vitamin B12 play in the body and what is the best way to get this vitamin?

Although most people get more than enough vitamin B12, there are exceptions that are deficient. These exceptions are mainly vegans and people who have problems with too much stomach acid. That is why we will show you in this section how you can best supply your body with vitamin B12.

When and for whom is it important to take vitamin B12?

Taking vitamin B12 is important for people who cannot get enough vitamin B12 because of diet or illness. We have put together a table for you here:

Type Why intake is important
vegan Vegans do not eat any animal products and must therefore take vitamin B12 supplements.
vegetarian Vegetarians do not eat meat and may have a vitamin B12 deficiency when breastfeeding or pregnant if there is an increased need for nutrients.
People with stomach problems If the gastric mucosa cannot produce the intrinsic factor (necessary protein) correctly, the vitamin B12 will not be taken correctly.
people with intestinal problems If the intestines are not working properly or parts of the intestines have been removed, there may be a vitamin B12 deficiency.

Healthy people are almost never affected by a vitamin B12 deficiency. In healthy people, vitamin B12 is not excreted in the digestion process, but is instead retrieved and stored. The best vitamin B12 suppliers are meat, fish and eggs, but all in moderation. Dairy products can also be on the daily menu.

What is the best way to take vitamin B12?

In the case of an existing deficiency, vitamin B12 can best be taken in tablets or by injection. The daily intake of vitamin B12 is 2.5 to 6 micrograms. Supplements or fortified foods can provide the best intake of vitamin B12. Unlike animal sources, they do not pose a threat to the human body from hormones, antibiotics and animal protein.

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Injections have the same long-term therapeutic results as vitamin B12 tablets or capsules. If intrinsic factor, the protein that helps absorb vitamin B12, is missing, vitamin B12 can be absorbed through the small intestine in high enough doses. Two different types of cobalamins are injected. The cyanocobalamin and the hydroxocobalamin. (4)

  • Cyancobalamin: An artificial form of vitamin B12 - it does not occur naturally.
  • Hydroxocobalamin: Natural form of vitamin B12, also found in nature.

The other advantage of injections over tablets is that the vitamin B12 is taken as a pure substance and is not combined with colourings, sweeteners or flavorings.

The syringe is injected intramuscularly. This means that the vitamin B12 is injected directly into the muscle. The risk of something going wrong is very high, so it is always advisable to consult a doctor when injecting for the first time.


There are many foods and vitamin tablets fortified with vitamin B12 these days. Quite a few tablets have been produced for meat eaters. Most are folic acid tablets with some vitamin B12. Sometimes the tablets are packaged in a gelatin shell and are non-vegetarian. (5)

Vitamin B12 can also be taken through the oral mucosa. There is a toothpaste that has been enriched with vitamin B12 and contributes to vitamin B12 supply. This type of intake is particularly recommended for gastrointestinal problems.

How long does it take for taking vitamin B12 to work?

Our body needs very little of vitamin B12. That's why he can't record a lot at once. It is therefore wiser to constantly consume smaller amounts of it. The body will excrete a lot at once.

A vitamin B12 tablet often contains between 500 and 1000 μg of vitamin B12 per tablet. That's why you don't have to take a pill every day. The human body is able to store vitamin B12 in the liver for a long time. If you take high doses once, you can cover your supply of vitamin B12 for a long time.

Blister pack with 3 different tablets per chamber

Vitamin B12 cannot be taken well without an intact intrinsic factor. (Image source: Simone van der Koelen / unsplash)

If you consume less than 5 μg of vitamin B12 at a time, the body is able to absorb about 60% of that dose. However, it is assumed that the intrinsic factor is sufficiently present and functioning.

However, if you consume 500 μg or more at once, the intrinsic factor no longer plays a major role, since at most about 1% is then absorbed by the body. Since vitamin B12 is water soluble, it does not harm the body if overdosed. The excess vitamin B12 is excreted through the bladder.

What amounts of vitamin B12 do people need?

Vitamin B12 is one of the vitamins that the human body cannot produce itself. The right amount of vitamin B12 has a positive effect on the body, so it is imperative to constantly cover the daily requirement. How much vitamin B12 you need per day is individual. Nevertheless, certain daily intakes are recommended for each age and life situation:

Old Vitamin B12 in µg/day
Babies 0 to 12 months 0.4 to 0.8 µg/day
Children 1 to 13 years 1.0 to 2.0 µg/day
Young people and adults from 13 to 65 years 3.0 µg/day
pregnant women 3.5 µg/day
breastfeeding 4.0 µg/day


If you forget to meet your daily requirement of vitamin B12, you can become deficient. It is good to know that it is not difficult to meet your daily requirement of vitamin B12. It is enough if you eat well and exercise enough. An overdose of vitamin B12 is very unlikely.

How to Get Enough Vitamin B12 Naturally?

Most vitamin B12 is found in animal products such as fish, meat and seafood. Eggs and dairy products also contain a small amount of vitamin B12. Generally speaking, it is not very difficult to cover everyday use.

In this table you will find further information on the vitamin B12 occurrence in the selected foods:

Groceries Vitamin B12 in µg per 100 grams
Brie 1.7µg/100g
lowfat quark 0.9µg/100g
oysters 15 µg/100g
veal kidney 28 µg/100g
egg 1.9µg/100g

You also have to be careful when preparing the meat. Excessive heat and storage times can damage the meat. That's why you should sear the meat carefully and prepare it gently.

What other B vitamins in your body are important to you?

Vitamins are vital for the body. Most vitamins cannot be produced by the body itself and must be obtained from food. A distinction is made between the fat- and water-soluble vitamins. In addition to vitamin B12, there are other vitamins that are in the vitamin B group.

  • Vitamin B1: This vitamin is also called thiamine. It is necessary for the maintenance of carbohydrate metabolism and the normal functioning of the nervous system.
  • Vitamin B2: Vitamin B2 is also called riboflavin. It is involved in the metabolic processes for energy production. It also helps in growth and blood formation.
  • Niacin: This vitamin is also called niacin. It is important for the normal functioning of the skin, the nervous system and the overall metabolism.
  • Pantothenic acid: Pantothenic acid is also called vitamin B5. It is involved in the metabolic processes for energy production.
  • Vitamin B6: Vitamin B6 is also called pyridoxine. It is important for the metabolism of proteins.
  • Biotin: Biotin is the vitamin B7. It is responsible for the metabolism of amino acids.
  • Folic Acid: Vitamin B9 is folic acid. This acid is important for blood formation and the development of the neural tube in an unborn child.

The group of B vitamins are all water-soluble vitamins. This means that the vitamins go directly into the blood via the small intestine and are needed there. If there is an excess of a vitamin, the body simply excretes it through the urine.


The side effects of a vitamin B12 deficiency can be uncomfortable. However, side effects from a vitamin B12 overdose are not very serious, nor are they life-threatening. Nevertheless, you should make sure that you take your daily ration of vitamin B12 and have yourself checked by a doctor if you feel unwell.

The treatment for a vitamin B12 deficiency is rather simple. You can easily take the daily requirement with tablets, capsules or a syringe. Anyone who takes in more than enough vitamin B12 will immediately excrete excess through the urine. Vegans and vegetarians in particular, as well as pregnant and breastfeeding women, should be aware that they should pay more attention to their vitamin B12 intake.


  1. Krautler B. Organometallic chemistry of b(12) coenzymes. Met Ions Life Sci. 2009;6:1-51. doi:10.1039/BK9781847559159-00001
  2. Okuda K. Discovery of vitamin B12 in the liver and its absorption factor in the stomach: a historical review. J Gastroenterol Hepatol. 1999;14(4):301-308.
  3. Owens JA, Scott JS, Cooksley WG. The retention of cobalamin by the liver in the cobalamin-deficient rat. Clin Sci (London). 1985;68(5):553-560. doi:10.1042/cs0680553
  4. Thakkar K, Billa G. Treatment of vitamin B12 deficiency-methylcobalamin? Cyancobalamins? Hydroxocobalamin?-clearing the confusion. Eur J Clin Nutr. 2015;69(1):1-2. doi:10.1038/ejcn.2014.165
  5. Rufenacht P, Mach-Pascual S, Iten A. Hypovitaminosis E12: challenge diagnostic et thérapeutique [Vitamin B12 deficiency: a challenging diagnosis and treatment]. Rev Med Suisse. 2008;4(175):2212-2217.
  6. Doreen Gille, Alexandra Schmid, Vitamin B12 in meat and dairy products, Nutrition Reviews, Volume 73, Issue 2, February 2015, Pages 106-115, https://doi.org/10.1093/nutrit/nuu011
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