Vitamin B12: what is it, benefits, side effects, dosage

Vitamin B12 is hugely important for your health because it helps build and maintain DNA and RNA, red blood cells, and nerve cells. Many different conditions can make you B12 deficient. But before you start popping B12 pills, learn about what might put you at risk for a deficiency, how much B12 you should take, and what the possible dangers of supplementation are.

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Vitamin B12, also known as cobalamin, is a nutrient in the B-vitamin complex. It’s made only by certain microorganisms, most of which can be found in the guts of ruminant (cud-chewing) animals, such as cattle and sheep (Ankar, 2021; Gille, 2015).

Vitamin B12 has the largest and most complex structure of all the vitamins (Linus Pauling, 2021). In food, it’s most significantly found in animal products, including fish, red meat, poultry, eggs, and dairy products. The bioavailability (how much enters the bloodstream) of vitamin B12 is about three times higher in dairy products than in meat, fish, or poultry (HHS, 2021).

In plants, some levels of vitamin B12 can be found in certain fermented beans, algae, and mushrooms (Linus Pauling, 2021). Many other plant foods don’t contain vitamin B12 unless it’s added to them. Vitamin B12-fortified foods can include breakfast cereals, vegan spreads, non-dairy milk, meat substitutes, and nutritional yeast. (But if you’re concerned about getting enough vitamin B12, check the labels on these foods—not all are fortified with B12).

Your body uses vitamin B12 for making DNA and RNA, red blood cells, and myelin, the protective sheath around nerve cells. Because it has so many vital functions, getting enough vitamin B12 is absolutely essential for your health (HHS, 2021).

If you eat a well-balanced diet, you likely don’t need to supplement with B12. Some people are at higher risk of developing a B12 deficiency, though. Here are some of the risk factors for vitamin B12 deficiency (USDA, 2019; Langan, 2017; HHS, 2021):

• Being a vegan or strict vegetarian

• Having had stomach or small intestine surgery

• Having a stomach and/or small intestine disorder that interferes with absorption of nutrients, such as pernicious anemia, inflammatory bowel disease, or celiac disease

• Having a Helicobacter pylori (H.pylori) infection

• Using the diabetes drug metformin for more than four months

• Using proton pump inhibitors (such as omeprazole and esomeprazole) or histamine H2 blockers for more than 12 months

• Being over age 60 (and especially over age 75)—Approximately 6% of adults younger than 60 years in the United States and the United Kingdom have a vitamin B12 deficiency, but the rate is closer to 20% in people over 60.

Routine testing for vitamin B12 deficiency isn’t typically recommended for people who don’t have one or more of these risk factors, though speak with your healthcare provider if you are concerned about your levels.

If your vitamin B12 blood levels are normal, then supplementing isn’t necessary. But if you have low levels of vitamin B12, the benefits of supplementing may include (HHS, 2021):

• Increasing energy by helping to form red blood cells

• In pregnant women, preventing major harm to the fetus

• Reducing the risk of macular degeneration, a disease of the eyes

• Improving your mood and helping with depression (Sangle, 2020)

• Improving your memory and slowing mental decline (Oulhaj, 2016)

Some studies have found that supplementing with B12 may increase your risk of colorectal cancer and lung cancer (Oliai Araghi, 2019; Fanidi, 2019). It’s still far from certain that this is the case, and more studies need to be done. It does mean, though, that to be on the safe side, it’s likely best to supplement with vitamin B12 only if you’ve been tested and found to have a deficiency. Speak with your healthcare provider to see what makes the most sense for your situation. If deemed necessary, you should take only the recommended amounts of B12 (see chart below).

Vitamin B12 is available as an over-the-counter (OTC) supplement and also as a prescription medicine (HHS, 2021).

OTC vitamin B12 supplements

The most common form of vitamin B12 in dietary supplements is cyanocobalamin. Other forms are adenosylcobalamin, methylcobalamin, and hydroxycobalamin.

Oral (swallowed) supplements are available as pills and tablets. There are also sublingual (under the tongue) B12 tablets, lozenges, liquids, and sprays. There’s no difference in effectiveness between the oral and sublingual forms of vitamin B12.

Vitamin B12 supplements are available in different formulations, too. You can get them in combination with other vitamins and minerals as multivitamin/mineral supplements, in combination with other B vitamins as B-complex supplements, and as stand-alone supplements that contain only vitamin B12.

• Multivitamin/mineral supplements usually contain 5–25 mcg of B12

• B-complex vitamin supplements generally contain 50–500 mcg of B12

• Stand-alone vitamin B12 supplements typically contain 500–1,000 mcg of B12

Vitamin B12 has a unique characteristic that makes it different from many other vitamins. Its bioavailability (the amount that gets into your bloodstream so your body can use it) from dietary supplements is about 50% higher than its bioavailability from food sources, whereas many other vitamins are better absorbed from food than supplements (HHS, 2021).

Prescription vitamin B12 supplements

Prescription vitamin B12 is usually injected directly into a muscle, bypassing your digestive system. Injections are often given to people with severe deficiencies due to conditions that make them unable to absorb B12 through their intestines (for instance, tropical sprue and pancreatic insufficiency). Prescription vitamin B12 is also available as a nasal spray.

Your vitamin B12 levels are determined by a blood (serum) test. The lab will show you as having levels that are normal, borderline, deficient, or elevated (Ankar, 2021; HHS, 2021):

• Normal levels are above 300 pg/mL

• Borderline levels are between 200 and 300 pg/mL

• Deficient levels are defined by most labs as levels lower than 200 pg/mL 

• Elevated levels are above 1000 ng/L (Lacombe, 2021)

If results aren’t clear after the first test, your healthcare provider may order further testing using different markers that may indicate a vitamin B12 deficiency. 

High levels of vitamin B12 are unusual, but they can indicate that something else is wrong physically.

Elevated vitamin B12 levels are linked with the presence of solid cancer tumors. High levels of vitamin B12 may also be linked with the presence of blood cell disorders, diabetes, heart failure, obesity, AIDS, or severe liver disease (Lacombe, 2021; Shipton, 2015).

How much vitamin B12 you need depends on your life stage and other factors, like whether you’re pregnant or breastfeeding. Most people get all the B12 they need from their daily diet unless they have one or more of the risk factors listed above.

No upper limit has been established for B12 supplementation. It has a low risk of toxicity, even at high doses, because it’s a water-soluble vitamin, meaning it doesn’t get stored in excessive amounts in the body (HHS, 2021). However, as mentioned above, some studies have found that taking B12 supplements may be linked to an increased risk of colorectal cancer and lung cancer. If you have one or more risk factors for deficiency, get tested for B12 deficiency before taking B12 supplements.

Most people in the United States get enough vitamin B12 from the foods and beverages they consume (unless they are vegans or strict vegetarians). If you have one or more risk factors for vitamin B12 deficiency, have a blood test done by a healthcare provider. Your provider will suggest the type and amount of B12 supplementation you need.

Be aware that B12 supplements can interact with certain medications, including metformin and some medications commonly taken for acid reflux (HHS, 2021). Be sure to tell your healthcare provider about any medications or additional supplements you’re taking.

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Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

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Current version

November 08, 2021

Written by

Alison Dalton

Fact checked by

Steve Silvestro, MD

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About the medical reviewer

Steve Silvestro, MD, Ro

Dr. Steve Silvestro is a board-certified pediatrician and Associate Director, Clinical Content & Education at Ro.