Well, doses over 800x the recommended daily intake have been shown to be safe for treating B12 deficiency , albeit with mild side effects. And since our bodies have the ability to store Vitamin B12 for years , even deficiency isn’t common.
But if we get too little Vitamin B12, we can be in serious trouble. But let’s back up a little.
Vitamin B12, also called Cobalamin, is essential for maintaining healthy red blood cells, keeping our nerve cells in great shape, DNA synthesis, preventing anemia, keeping our energy up, and many other benefits. 
There are a few caveats to B12’s benefits, however. Vitamin B12 interacts with both Vitamin C and Folate (you may know the synthetic form as Folic Acid), something we’ll look into later in the article. And pre-existing conditions, such as pernicious anemia, may lead to deficiency, even if your diet contains enough Vitamin B12.
Vegetarians and vegans take notice: you may need to take Vitamin B12 supplements, since plant-based foods don’t contain the nutrient, unless they’re specifically fortified with it. On the other hand, diets containing poultry, beef, eggs, dairy products, fish and fortified cereals are rich sources of Vitamin B12.
In the end, dosage comes down to lifestyle, genetics and blood levels. Rootine carefully inspects all three to figure out your appropriate Vitamin B12 needs, and ensures any interactions with other vitamins, as well as any conditions, are accounted for.
Let’s dive further into what Vitamin B12 benefits.
What is Vitamin B12 good for?
Besides maintaining red blood cell health, helping create DNA, and ensuring you don’t get anemia, other Vitamin B12 benefits include helping with mood disorders and even memory.
A study from the Open Neurology Journal found that patients with depression and low Vitamin B12 levels demonstrated improved depressive symptoms when taking both antidepressants and the vitamin, as opposed to groups who only took antidepressants. Vitamin B12 deficiency was actually seen to increase the risk of serious depression in another study.
Even in the absence of deficiency, Vitamin B12 has shown to improve memory. One study found that Vitamin B12 levels that are low - not deficient - can create poor memory performance.
How much Vitamin B12 do you need?
For both women and men, the recommended dose by the Food and Nutrition Board is 2.4 mcg daily, 2.6 mcg if you’re pregnant and 2.8 mcg if lactating.
We recommend taking these doses with a grain of salt - they’re meant to be averages across an entire population. It’s like wearing one-size-fits-all clothes: it’s much better to get something that fits your body, rather than hundreds of millions of people.
Before we dive into the side effects of Vitamin B12 deficiency and overdose, it’s worth noting that Vitamin C - especially in larger doses - may destroy Vitamin B12, when the two vitamins are taken together, though this is a debated topic. Spacing either vitamin apart by a few hours is recommended.
What are the symptoms of low Vitamin B12 levels?
There are some pretty nasty side effects that come with Vitamin B12 deficiency, including fatigue, weakness, appetite loss, weight loss, constipation and anemia. The grocery list of symptoms doesn’t end there. Lacking enough Vitamin B12 has also been associated with numbness and tingling in hands and feet, changes in mood and memory, and other neurological symptoms .
There are a few ways you can be deficient. As we mentioned at the start of the article, vegetarians and vegans should lean on Vitamin B12 supplements as their diets don’t contain adequate levels of the nutrient. Those with pernicious anemia are at an even greater risk of deficiency.
Pernicious anemia destroys vital stomach cells that produce a protein called “intrinsic factor,'' which normally combines with Vitamin B12 and helps it get absorbed in the small intestine. Even with adequate levels of Vitamin B12 in their diet, those with pernicious anemia might still be deficient .
Large Vitamin B12 doses are needed to get B12 levels normal again if you’re deficient. If the deficiency goes untreated, permanent nerve damage may occur .
Vitamin B12 is especially important for women who are pregnant. One study showed that women with Vitamin B12 levels under a certain amount were 3-5x more likely to give birth to a child with birth defects, when compared with those with adequate levels .
Can you overdose on Vitamin B12?
The side effects of a Vitamin B12 overdose include headache, dizziness, anxiety, nausea and vomiting .
Your body actually prevents you from absorbing too much Vitamin B12. Just 56% of a 1 mcg oral dose gets absorbed, and this percentage declines as you go above 1 mcg. Only about 10 mcg out of 500 mcg is absorbed in healthy people .
Though the recommended daily dose is 2.4 mcg, doses of 2,000 mcg have safely been given to those with a Vitamin B12 deficiency .
What foods have Vitamin B12?
As we mentioned above, animal products contain adequate levels of Vitamin B12. Vegetarians and vegans, and anyone really, can also get Vitamin B12 from fortified cereals .
The USDA National Nutrient Database for Standard Reference contains a very comprehensive list (212 pages) of various foods and their Vitamin B12 values. It’s worth exploring a bit to get a sense of what foods contain high levels of B12. Some foods high in Vitamin B12 include:
- 4 oz of New Zealand beef liver has nearly 40x your daily requirement of Vitamin B12
- One cup of soy milk with added nutrients contains our daily requirement of B12 (2.43 mcg)
- In the reference, 80 pages from the standard reference contain foods with absolutely no Vitamin B12 - many of these are tasty snacks!
The Dietary Guidelines for Americans  on Health.gov recommends fish and red meat as good sources of Vitamin B12, as well as poultry and eggs. Diets should also include “a variety of vegetables, fruits, whole grains, fat-free or low-fat milk and milk products, and oils.”
Vitamin B12 and Folate
We don’t see enough easy-to-read articles diving into the interaction between Vitamin B12 and Folate, so we decided to add the following sections. It gets a bit sciency, but we promise it’s still a fun read and you’ll come out smarter in the end.
Vitamin B12 plays an important role in Folate metabolism. The “Active” form of Folate interacts with B12 to lower homocysteine in the blood (to get a quick grasp on the various forms of Folic Acid, read our article on the nutrient). High levels of homocysteine may lead to heart disease . That’s why when talking about adequate Folate supply, Vitamin B12 is always part of the equation.
What are the best Vitamin B12 types?
Vitamin B12 comes in three main forms:
- The natural form, called Methylcobalamin (“Active” B12) and found in foods.
- The synthesized form, called Cyanocobalamin (“Inactive” B12) and used in most supplements.
- Adenosylcobalamin (another Active form of B12).
So which type is best? There are good arguments for and against each form of B12.
Methylcobalamin: the natural form of Vitamin B12
This is the form that interacts directly with the Active form of Folate to improve heart health. Any other form of B12 first needs to be converted to the Active form before it can carry out its positive effects alongside Folate. As it’s the natural form, Methylcobalamin is preferred by some as their choice B12 supplement.
Cyanocobalamin: the synthesized form of Vitamin B12
Cyanocobalamin is the main B12 form used in supplements, as it has a number of advantages over the natural form:
- The body can convert (the Inactive) Cyanocobalamin to the Active form efficiently, so it supplies adequate amounts of B12 to the body if used correctly .
- This form is significantly more stable in the form of a supplement.
- This form is absorbed at a higher rate than the natural form .
- This form does not only convert to the Active form Methylcobalamin, it also converts to Adenosylcobalamin, an essential form of Vitamin B12. This is why the “natural” form is less suitable to treat medical B12 deficiency.
There are also some criticisms circling Cyanocobalamin:
- It’s not the natural form the body is used to.
- It contains a Cyanide molecule, which is actually a very potent poison (see the Rootine Science box below).
Rootine Science: Is there poison in my Vitamin B12 supplements?
At Rootine, we are committed to clearing up any confusion around nutrients, and that means closely observing criticisms in the media and the scientific community. The argument that there is literal poison in the Cyanocobalamin form of Vitamin B12 is thrown around a lot. The recommended daily allowance (RDA) for Vitamin B12 is 2.4 mcg per day. Around 0.14% of the B12 molecule is a Cyanide molecule, so an ingestion of 2.4 mcg of Vitamin B12 in the Cyanocobalamin form would expose the body to 0.003 mcg of Cyanide.
The “normal” amount of Cyanide in our body is considered to be around 1,600 mcg, which is continuously processed and neutralized from the body. If you would take 2.4 mcg of Cyanocobalamin every day, it would take you 26 years to reach the amount of Cyanide that is contained within a single almond .
While the argument for the natural form of Vitamin B12 is understandable, you should be aware that Methylcobalamin is not converted into other forms of B12, which are also important to your health. If you’re not taking Cyanocobalamin, you can seek out additional supplements that ensure you have an adequate nutrient supply (look for Adenosylcobalamin and Hydroxocobalamin).
At Rootine, after assessing your lifestyle, your genetic factors and your blood levels, we ensure you have all the necessary forms of Vitamin B12, as well as other essential nutrients. To learn more, see our FAQs Page.
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6. Stabler, Sally P. Vitamin B12 Deficiency and Depression in Physically Disabled Older Women: Epidemiologic Evidence From the Women’s Health and Aging Study. American Journal of Psychiatry, vol. 157, pp. 715-721.
7. Köbe, Theresa A. et al. Vitamin B-12 concentration, memory performance, and hippocampal structure in patients with mild cognitive impairment, The American Journal of Clinical Nutrition, vol. 103, issue 4, pp. 1045–1054.
8. Institute of Medicine. Food and Nutrition Board. Dietary Reference Intakes: Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington, DC: National Academy Press, 1998.
14. Molloy, Anne M et al. “Maternal vitamin B12 status and risk of neural tube defects in a population with high neural tube defect prevalence and no folic Acid fortification”, Pediatrics, vol. 123,3 (2009), pp. 917-923.
21. Obeid, Rima et al. “Cobalamin coenzyme forms are not likely to be superior to cyano- and hydroxyl-cobalamin in prevention or treatment of cobalamin deficiency” Molecular nutrition & food research vol. 59,7 (2015), pp. 1364-1372.
22. J. F. Adams, Sheila K. Ross, L. Mervyn, K. Boddy & Priscilla King. Absorption of Cyanocobalamin, Coenzyme B12, Methylcobalamin, and Hydroxocobalamin at Different Dose Levels, Scandinavian Journal of Gastroenterology, vol. 6:3 (1971), pp. 249-252.