Vitamin B12 supplements are popular among people who want to increase their energy level, endurance and sports performance. Even though the recommended dietary allowance (RDA) for B12 is only 2.4 micrograms (mcg), some supplements provide 10,000 mcg per serving, or 416, 666 percent of the RDA. That amount of B12 daily for a long time can have potentially serious effects.
What Is B12?
B12 is a critical nutrient in the formation of red blood cells. Red blood cells contain hemoglobin, a protein that carries oxygen throughout the body. Without enough — or properly formed — red blood cells, the body can't get the oxygen it needs. This results in a condition called anemia, which can cause fatigue, weakness, loss of appetite and depression.
In addition, vitamin B12 is required for neurological function and DNA creation, and it's also involved in metabolism and controlling blood levels of an amino acid called homocysteine.
Risks of Too Much B12
As a water-soluble nutrient, excess B12 isn't stored in the body. It must be replaced through food or supplements each day to keep blood levels at a normal level. Because blood levels can't build up, there's low risk of toxicity. For this reason, the National Academy of Medicine did not establish an upper limit (UL) for B12, noting that no negative effects are associated with excess B12 from both food and supplements in healthy adults.
However, a few studies suggest B12 — alone or in combination with other B vitamins — may have negative consequences for special populations. A 2010 study in JAMA found that doses of 1 mg B12 per day, in combination with vitamin B6 and folic acid, for six years in patients with diabetic neuropathy increased the risk of vascular events, including heart attack and stroke.
A study published in 2018 in Paediatric and Perinatal Epidemiology showed that very high maternal blood levels of B12 at birth increased the risk of autism spectrum disorder. Another study in 2017 in Journal of Clinical Oncology reported that long-term use of high-dose B12 and B6 was associated with a 30- to 40-percent increase in lung cancer risk in men.
Benefits of B12
Because of its role in healthy red blood cells and oxygen transport, people often think that taking megadoses will improve energy and endurance. However, B12 will have these benefits only if you're already deficient in the vitamin.
High levels of homocysteine are associated with low B12 and increased risk of heart disease and cognitive decline. However, while a high dose of B12 does reduce homocysteine levels, it hasn't been shown to reduce the risk of heart attack or to improve cognitive function.
Getting Enough B12
B12 is primarily found in animal foods. Because of that, many vegetarians and vegans find it difficult to get enough of the vitamin from their diets, so supplementation may be necessary. The body's ability to absorb B12 also decreases with age, so older adults may need a B12 supplement. Otherwise, B12 is abundant in foods, and the RDA can easily be met by eating fish, meat, poultry, eggs and dairy products.
- NIH: Vitamin B12
- University of Rochester Medical Center: What Are Red Blood Cells?
- Now Foods: Methyl B-12 10,000 mcg Lozenges: Product Description
- WebMD: Heart Disease and Homocysteine
- JAMA: Effect of B-vitamin therapy on progression of diabetic nephropathy
- Paediatric and Perinatal Epidemiology: Maternal Multivitamin Intake, Plasma Folate and Vitamin B12 Levels and Autism Spectrum Disorder Risk in Offspring
- Journal of Clinical Oncology: Long-Term, Supplemental, One-Carbon Metabolism-Related Vitamin B Use in Relation to Lung Cancer Risk in the Vitamins and Lifestyle (VITAL) Cohort
- American Journal of Clinical Dermatology: A review of vitamin B12 in dermatology.
- ISRN Psychiatry: The Effect of Methylated Vitamin B Complex on Depressive and Anxiety Symptoms and Quality of Life in Adults with Depression
- Weil: Too Much Vitamin B12?