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Vitamin B12 may be one of the most important vitamins our body needs. But that doesn’t mean megadoses of the nutrient are a good thing. Most healthy people get enough vitamin B12 from food.

About 1%-2% of people have a confirmed vitamin B12 deficiency, says M. Elizabeth Swenor, DO, who leads Henry Ford Health System’s Lifestyle, Integrative, and Functional Medicine in Detroit. If you're one of these people, you may benefit from higher doses of B12 but only under the supervision of a health care professional.

What Is Vitamin B12?

B12 is one of eight B vitamins. It helps produce the red blood cells that carry oxygen throughout our body. It’s also critical for nerve function, energy release, and even contributes to DNA formation, says registered nutritionist Lauri Wright, PhD, a national spokesperson for the Academy of Nutrition and Dietetics.

Your body doesn’t naturally make vitamin B12. That means you have to get it from food or supplements. Once ingested, the vitamin combines with a protein in your stomach called intrinsic factor. “Intrinsic factor is like a taxicab that transports B12 from the stomach to the part of the bowel where it’s absorbed,” Swenor says.

How Much Vitamin B12 Do You Need?

Most healthy adults only need a small amount of B12 -- 2.4 micrograms (mcg). B12 is plentiful in animal products like meat and fish, milk and other dairy products, eggs and, clams, which have a huge amount of B12. Some breakfast cereals, breads, and grains are also fortified with B12.

Three ounces of tuna will provide all the vitamin B12 you need for the day. Two eggs will give you half of the recommended daily amount, Wright says.

Vitamin B12 Deficiency

People most at risk for vitamin B12 deficiency include those over the age of 65, vegans and vegetarians (who often don’t eat enough animal products), people who have gastrointestinal diseases like Crohn’s disease or celiac disease, and anyone who has had part of their gastrointestinal tract removed through gastric bypass or another type of surgery, Wright says.

Symptoms of B12 deficiency, which can develop over years, may include fatigue, pale skin, numb or tingly hands or feet, and confusion. Left untreated, too little vitamin B12 can ultimately lead to nerve damage, ataxia (difficulty with walking and balance), pernicious anemia, heart failure, and even gastric cancer, Swenor says.

If you belong to any high-risk groups, talk to your doctor about a blood test to make sure you have enough vitamin B12.

“If a problem exists, we try to tweak the diet,” says Wright, who is also chair of the department of nutrition and dietetics at the University of North Florida. If that's not enough, you may need a supplement. Initially, the dose may be as high as 500 mcg. That is lowered to a maintenance dose of 100-200 mcg, Wright adds. If a person doesn't have enough intrinsic factor, you may need an injection of vitamin B12.

Myths About B12 Supplements

While too little B12 is associated with many health problems, adding large doses of supplements probably won't fix the issues. Despite scientific evidence, some myths about B12 and common health conditions continue:

  • Alzheimer’s. Low B12 levels have been linked with memory problems and even dementia but studies have not shown any B12 benefit in people with one of these conditions, even in megadoses of 1,000 micrograms. “We are a long way from saying vitamin B12 can improve Alzheimer’s or even memory,” Wright says. “Animal studies report a slight improvement in memory, but there’s no evidence in human studies.”
  • Cancer. Different studies have linked both high and low levels of B12 with cancer. That basically means no one knows whether the vitamin helps or hurts, or even if deficiencies increase your risk.
  • Heart disease and stroke. Higher levels of the amino acid homocysteine may be associated with a higher risk of heart attack and stroke. B12 may lower homocysteine levels which could contribute to a lower odds of heart disease, but only in people who have a diagnosed B12 deficiency, Swenor says.
  • Infertility. B12 has been shown to improve sperm’s ability to move. But B12 supplements aren’t likely to help if there’s no deficiency, Swenor says.
  • Age-related macular degeneration. Studies have suggested that adding folic acid, B6, and B12 may reduce the chance of age-related macular degeneration, an eye condition that gets worse over time. But researchers have stopped short of recommending supplements for it.
  • Eczema. The American Academy of Dermatology points out one study has found that B12 cream reduced eczema in adults. But a large study found no link between B12 levels and risk of eczema. “Other B vitamins are involved with skin health but not so much B12,” Wright says.
  • Sickle cell anemia. B12 supplements often don't help and may even hurt people with sickle cell anemia, Wright says. She adds that you should take supplements only after talking with a medical team.
  • Energy. “Vitamin B12 can improve energy, concentration, memory, and mood only in individuals that have an existing B12 deficiency,” Swenor says. “There is not any clinical evidence to suggest that it can boost energy in people who already have ample stores of B12.”
  • Weight loss. If megadoses of B12 have any benefit on weight loss, it’s probably a placebo effect, Wright says. People 65+Vegetarians & vegansPeople who have gastrointestinal diseasesPeople who have had parts of their intestines surgically removed

Is Vitamin B12 Safe? How Do Other Drugs Affect B12?

Even high doses of B12 don’t pose any significant health risk. Because it's water soluble, any excess will be flushed out in your urine, Wright says.

Some medications can reduce B12 levels. These include:

  • Heartburn drugs and stomach-acid reducers like omeprazole (Prilosec) and ranitidine (Zantac)
  • Metformin, a diabetes medication
  • Vitamin C
  • Anti-seizure medications like phenytoin (Dilantin)
  • Colchicine, a gout medication
  • Bile acid sequestrants to lower cholesterol like colestipol (Colestid), cholestyramine (Questran), and colesevelam (Welchol)
  • Tetracycline, an antibiotic

Show Sources


Lauri Wright, PhD, RDN, national spokesperson, Academy of Nutrition and Dietetics and chair, department of nutrition and dietetics, University of North Florida.

M. Elizabeth Swenor, DO, in charge of Lifestyle, Integrative, and Functional Medicine, Henry Ford Health System, Detroit.

Mount Sinai: “Vitamin B12 (Cobalamin).”

Merck Manual: “Vitamin B12 Deficiency.”

National Institutes of Health Office of Dietary Supplements: “Vitamin B12.”

Cleveland Clinic: “The Best Sources of Vitamin B12.”

Harvard Medical School: “Vitamin B12 deficiency can be sneaky, harmful.”

National Heart, Lung, and Blood Institute: “Pernicious Anemia.”

International Journal of Cancer: “Biomarkers of folate and vitamin B12 and breast cancer risk: report from the EPIC cohort.”

Memorial Sloan Kettering Cancer Center: “Vitamin B12.”

National Health Service UK: “Complications - Vitamin B12 or folate deficiency anaemia.”

Archives of Internal Medicine: “Folic Acid, Vitamin B6, and Vitamin B12 in Combination and Age-related Macular Degeneration in a Randomized Trial of Women.”

American Academy of Dermatology: “Can oils, probiotics, or vitamins heal eczema?”

Dermatology: “Association of Serum Vitamins with Eczema in US Adults (NHANES 2005–2006).”

Mayo Clinic: “Vitamin B-12.”