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If you have any medical questions or concerns, please talk to your healthcare provider. The articles on Health Guide are underpinned by peer-reviewed research and information drawn from medical societies and governmental agencies. However, they are not a substitute for professional medical advice, diagnosis, or treatment.
Many promote vitamin B12 as a great way to boost energy, but taking too much B12 is not the way to go about fixing that afternoon slump. That’s not to say vitamin B12 isn’t important. It is. In fact, those who are low in vitamin B12 can benefit from supplementing with higher doses of this water-soluble vitamin (vitamins that your body doesn’t store in its tissues). But unless your healthcare provider has told you otherwise, you’re probably not lacking in the vitamin, as most Americans do not have a deficiency (Langan, 2017).
So, how much vitamin B12 is too much? Let’s look at what higher doses of this essential vitamin can do to the body and why it’s best to avoid taking in more of the vitamin than necessary.
Can you overdose on vitamin B12?
Overdosing on vitamin B12 is unlikely. Because vitamin B12 (cobalamin) is a water-soluble vitamin and your body excretes it daily in your urine, it doesn’t build up in the body like fat-soluble vitamins—like vitamin D—do. A tolerable upper intake level (UL)—the amount you can safely take without risk of severe side effects—was never set for it because it has a low potential for toxicity, and the body does not store it in excess (NIH-a, 2021).
Are high doses of vitamin B12 harmful?
It’s recommended adults get at least 2.4 mcg of vitamin B12 daily. That being said, it’s very easy to take two supplement gummies and consume 125,000% of the recommended dietary allowance (RDA). Extra strength B-12 supplements offer this amount—3,000 mcg—in one serving. If you think 125,000% seems excessive, you may be right (NIH-a, 2021).
Fortunately, even at large doses, vitamin B12 is generally considered safe, at least in the short term (more on that below). Oral B12 supplements often contain higher levels than the recommended daily allowance (RDA). The reason for higher amounts is that the body doesn’t absorb most of it. For example, if you take a low dose supplement (less than 1-2 mcg), your body absorbs only about half of it. If you take a high dose (1,000 mcg), your body absorbs only about 1% of it (NIH-a, 2021).
You should tailor a B12 supplement to meet your needs and not overdo it because the liver stores B12, and the kidneys send excess vitamin B12 out through urine. So, unless you’re deficient, megadoses of vitamin B12 may be just passing through your body and making some organs work harder (NIH-a, 2021). Immediate reactions to high-dose B12
Vitamin B12: what is methylcobalamin?
Healthcare professionals typically use vitamin B12 amounts of 1,000 mcg and higher to treat a deficiency. Serious reactions, like allergic reactions, are generally rare but can occur with vitamin B12 injections (Amin, 2021).
Injections bypass the digestive system and are used for those who need immediate B12 or have a more challenging time absorbing nutrients. These rare allergic reactions can include itching or rash, fever, swollen tongue, and other symptoms (Vassavada, 2021; Brescoll, 2015).
Possible long-term risks of vitamin B12
Most side effects of high doses of B12 are rare and temporary. But some studies have led to questions about high-level B12 doses in those not vitamin B12 deficient or at risk of a deficiency.
Results from a 2017 study show an association with consuming high-dose B6 and B12 vitamin supplements over 10 years with an increase in lung cancer risk in men, especially smokers (Brasky, 2017).
Another study in the Journal of the American Medical Association notes that among those with diabetic nephropathy (loss of kidney function due to diabetes), combining vitamin B6 (25 mg) and folic acid (2.5 mg) (B-complex vitamin) with 1,000 mcg of oral B12 per day for six years increases the risk of cardiovascular events, including heart attack and stroke. It also decreases kidney function (House, 2010).
Who needs vitamin B12?
Can vitamin B12 injections boost your health?
Even those who get enough in their diet can be deficient in the vitamin if they have certain medical conditions or take specific medications. For example, those who follow vegan or vegetarian diets or are over the age of 60 may need supplementation (Pawlak, 2014; Langan, 2017).
If you have symptoms like weakness or fatigue, heart palpitations, confusion, balance problems, infertility, pale skin, or loss of appetite, reach out to your healthcare provider to see if you should undergo a B12 screening, which involves a simple blood test. These are symptoms of pernicious anemia, which occurs when the body doesn’t have enough vitamin B12 to support healthy red blood cell production (NIH-b, 2021).
Like other vitamins, the goal is to balance vitamin B12. To do this, it’s best to discuss all supplements and medical conditions with your healthcare provider.
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- Ankar, A. (2021). Vitamin B12 deficiency. [Updated Jun 7, 2021]. In: StatPearls [Internet]. Retrieved on Nov. 9, 2021 from https://www.ncbi.nlm.nih.gov/books/NBK441923/
- Brasky, T. M., White, E., & Chen, C.-L. (2017). Long-term, supplemental, one-carbon metabolism–related vitamin B use in relation to lung cancer risk in the vitamins and lifestyle (VITAL) cohort. Journal of Clinical Oncology, 35(30), 3440–3448. doi: 10.1200/jco.2017.72.7735. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5648175/
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- House, A. A., Eliasziw, M., Cattran, D. C., Churchill, D. N., Oliver, M. J., Fine, A., et al. (2010). Effect of B-vitamin therapy on progression of diabetic nephropathy. JAMA, 303(16), 1603. doi: 10.1001/jama.2010.490. Retrieved from https://jamanetwork.com/journals/jama/fullarticle/185758
- Lacombe, V., Chabrun, F., Lacout, C., Ghali, A., Capitain, O., Patsouris, A., et al. (2021). Persistent elevation of plasma vitamin B12 is strongly associated with solid cancer. Scientific Reports, 11(1). doi: 10.1038/s41598-021-92945-y. Retrieved from https://www.nature.com/articles/s41598-021-92945-y#
- Langan, R. C., & Goodbred, A. J. (2017). Vitamin B12 deficiency: Recognition and management. American Family Physician, 96(6), 384–389. Retrieved from https://www.aafp.org/afp/2017/0915/p384.html
- Pawlak, R., Lester, S. E., & Babatunde, T. (2014). The prevalence of cobalamin deficiency among vegetarians assessed by serum vitamin B12: A review of literature. European Journal of Clinical Nutrition, 68(5), 541–548. doi: 10.1038/ejcn.2014.46. Retrieved from https://www.nature.com/articles/ejcn201446
- Raghavan, R., Riley, A. W., Volk, H., Caruso, D., Hironaka, L., Sices, L., et al. (2017). Maternal multivitamin intake, plasma folate and vitamin B12LEVELS and autism spectrum disorder risk in offspring. Paediatric and Perinatal Epidemiology, 32(1), 100–111. doi: 10.1111/ppe.12414. Retrieved from https://pubmed.ncbi.nlm.nih.gov/28984369/
- Shipton, M. J., & Thachil, J. (2015). Vitamin B12 deficiency – A 21st century perspective. Clinical Medicine, 15(2), 145–150. doi: 10.7861/clinmedicine.15-2-145. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4953733/
- U.S. Department of Health and Human Services (NIH-a). (n.d.). Office of dietary supplements – vitamin B12. NIH Office of Dietary Supplements. Retrieved from https://ods.od.nih.gov/factsheets/VitaminB12-Consumer/
- U.S. Department of Health and Human Services (NIH-b). (n.d.). Office of dietary supplements – vitamin B12. Retrieved November 1, 2021 from https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/
- U.S. Department of Agriculture (USDA). (2021). Vitamin B12. Retrieved November 9, 2021 from https://www.ars.usda.gov/northeast-area/beltsville-md-bhnrc/beltsville-human-nutrition-research-center/docs/vitamin-b12/
- Urbanski, G., Hamel, J.-F., Prouveur, B., Annweiler, C., Ghali, A., Cassereau, J., et al. (2020). Strength of the Association of elevated vitamin B12 and solid cancers: An adjusted case-control study. Journal of Clinical Medicine, 9(2), 474. doi: 10.3390/jcm9020474. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7073937/
- Vasavada, A. (2021). Cyanocobalamin. [Updated Jul 25, 2021]. In: StatPearls [Internet]. Retrieved November 1, 2021 from https://www.ncbi.nlm.nih.gov/books/NBK555964/#
Felix Gussone is a physician, health journalist and a Manager, Medical Content & Education at Ro.