To B12 or Not to B12? That Is the Question

Vitamin B12 (cobalamin) is essential for energy production, healthy blood, and nervous system functions, with recent research shedding interesting new insights into its role in cognition. Findings from 2025, for instance, suggest that even in healthy older adults with adequate B12 levels, lower amounts of this vitamin may have subtle but nonetheless detrimental effects on cognition (see box at end).

But what exactly is vitamin B12, why is it so important, who needs to pay extra attention to getting enough, and who might benefit from supplementation? Here’s a look at the science, the sources, and other things to know about this essential nutrient.

B12 basics

Vitamin B12 is a water-soluble nutrient that’s vital for many body functions, including DNA synthesis, the proper functioning of the nervous system (such as forming the myelin sheath that surrounds nerves), and the formation of cells produced in the bone marrow, especially red blood cells. It’s naturally found in animal-derived foods such as seafood, beef, chicken, eggs, and dairy, as well as in fortified foods, including breakfast cereals, nutritional yeast, dairy alternatives (like soy and almond milks), and certain breads. Algae and fermented foods, however, are not considered reliable sources of B12 for vegans.

The Recommended Dietary Allowance (RDA) for adult men and women, as well as the Daily Value (DV, as seen on food labels), is 2.4 micrograms (mcg) of B12 a day, an amount found, for instance, in about two cups of milk. In general, just one serving of fish or meat, or two servings of a fortified product, is enough to meet the RDA. (See chart of top B12 food sources.)

B is complex

The metabolism of B12 is quite complex. To put it simply, the B12 in meat is bound to protein, and gastric acids and digestive enzymes in the stomach are needed to break that attachment. The stomach also makes a protein called intrinsic factor, which attaches to B12 and carries it to the small intestine where it’s absorbed. In contrast, the B12 found in supplements is not bound to protein but still needs intrinsic factor to aid its absorption in the small intestine.

Although our bodies—more precisely, the bacteria in the colon (large intestine)—make vitamin B12, it’s not enough to meet our needs. That’s because B12 is not absorbed into the bloodstream from this part of the gut. To maintain adequate blood levels, you must get B12 from outside sources.

Who needs a B12 boost?

The body stores B12, primarily in the liver. In fact, so much can be stored that if a person stops consuming B12 from foods or supplements, it would take about two to five years to become deficient. Still, certain people are at higher risk of B12 deficiency for various reasons:

• Older adults, typically those over 50. With aging, the cells in the stomach lining change and produce less stomach acid, which in turn can hinder the body’s ability to absorb B12. Many older people also take medications that affect B12 absorption, such as proton-pump inhibitors (for acid reflux) and metformin (for diabetes).
• Vegetarians and vegans. Because vegetarians and vegans avoid many if not all animal foods, they get few or no natural sources of B12 and should focus on fortified products. It’s important to check the labels to make sure B12 has been added (not all nondairy milks, for instance, are fortified). But even if vegetarians/vegans consume fortified foods, some may still fall short of the RDA.
• People with certain medical conditions that affect the body’s ability to absorb B12, including diseases that reduce stomach acid or intrinsic factor production, such as atrophic gastritis or pernicious anemia—as well as conditions that cause damage to the end portion of the small intestine (the distal ileum) where B12 is absorbed (for instance, in Crohn’s disease and sometimes in celiac disease).
• People who have undergone gastric bypass surgery for weight loss. The surgery can result in a decrease in stomach acid needed for B12 absorption.

Which B12 to buy?

Supplements contain various forms of B12. Most common is cyanocobalamin, a synthetic form that’s cheapest and most stable. Natural forms of B12, like methylcobalamin and adenosylcobalamin, are also available in some supplements. There’s debate about whether one form—synthetic or natural—is better than the other in terms of bioavailability, with some studies finding that natural forms are more bioavailable but others showing minimal differences among most people. The NIH Office of Dietary Supplements maintains that there is no difference in absorption between them.

Oral supplements vs. injections vs. sublingual?

In addition to taking B12 supplements, B12 may be given by injection or in sublingual form.

Intramuscular injections are typically prescribed for individuals who are severely deficient—or those who are not getting enough B12 through diet or have a health condition such as pernicious anemia or Crohn’s disease where they have problems absorbing B12. Protocols vary but typically the injections are done once a week for a month and then once a month or every other month. Depending on the cause of the deficiency, injections may be needed for anywhere from a few months to the rest of one’s life.

Sublingual B12—taken under the tongue, where it dissolves—can, depending on the dose, be over the counter or by prescription. Some people prefer this route if, for example, they can’t swallow pills or don’t want injections. Because B12 is absorbed directly into the blood this way, sublingual administration bypasses any gut-related issues. But sublingual B12 hasn’t been as extensively studied as oral and intramuscular dosing.

If you’re deficient, your healthcare provider can recommend the best method of delivery and dose for you and check your blood B12 levels periodically. B12 injections, sublingual B12, and single B12 supplements provide higher doses (often 1,000 micrograms) than what you’d find in a multivitamin (which typically has up to 25 mcg of B12) or in a B-complex supplement (which can have up to 500 mcg or so).

When stores run dry

B12 deficiency is most notably associated with megaloblastic anemia, a specific type of anemia where red blood cells are abnormally large, making them ineffective at carrying oxygen to the body’s cells; this can also cause neurological problems, like peripheral neuropathy. Other consequences of B12 deficiency are fatigue, palpitations, and tingling or numbness in the hands or feet due to nerve damage.

Low B12 can also contribute to cognitive decline. However, more research is needed to confirm whether B12 levels affect cognition even within the normal range (see box).

Can you get too much? B12 is not considered toxic even in high amounts, as any excess is typically excreted in urine. Thus, the National Academy of Sciences has not established an upper limit for B12.

BOTTOM LINE: Your best bet for getting enough B12 naturally is to eat animal foods, but if you forgo them, fortified products are a reasonable alternative. Older people should consult their healthcare provider to check their blood level periodically and discuss whether they should take a supplement and in what form and how much. Healthy vegetarians/vegans who consume fortified foods should be able to get enough, but if they are concerned about their B12 status, they also should talk to their healthcare provider. Some hematologists recommend a B12 supplement when blood levels are at the lower end of “normal.”