The Side Effects of Too Much Vitamin B12

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.

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.

Read more: How Much Vitamin B12 Should a Woman Get?

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.

Excess B12 may also affect the skin, causing acne and rosacea, according to a 2015 review in the American Journal of Clinical Dermatology.

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.

In combination with other B vitamins, B12 may play a role in reducing symptoms in adults with depression, according to a 2013 study in ISRN Psychiatry.

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.

Read more: Symptoms of Low B12