Vitamin B12 is a water-soluble nutrient your body needs to make red blood cells, build DNA, and keep your nervous system working properly. Adults need about 2.4 micrograms per day, an amount most people get from animal-based foods, though deficiency is more common than you might expect.
What B12 Does in Your Body
B12 plays three major roles. First, it’s essential for producing healthy red blood cells. Without enough of it, your body makes red blood cells that are abnormally large and can’t carry oxygen efficiently, a condition called megaloblastic anemia. Second, B12 is required for DNA synthesis in every cell. Third, it supports the protective coating around your nerves (called the myelin sheath), which is why a deficiency can cause neurological problems long before blood tests look abnormal.
At the molecular level, B12 helps convert a compound called homocysteine into methionine, an amino acid your body uses as a building block for proteins and as a chemical switch that turns genes on and off. When B12 is low, homocysteine builds up in the blood, which has been linked to cardiovascular risk.
How Your Body Absorbs It
B12 absorption is surprisingly complex, which is part of why deficiency happens even in people who eat plenty of it. When you eat B12-containing food, stomach acid first separates the vitamin from the protein it’s bound to. Cells in your stomach lining produce a protein called intrinsic factor, which latches onto the freed B12. This B12-intrinsic factor pair then travels to the lower part of the small intestine, where it’s absorbed into the bloodstream.
If any step in this chain breaks down, you can become deficient even with adequate intake. People with low stomach acid (common in older adults), those who’ve had gastric surgery, and people with conditions affecting the small intestine like Crohn’s disease all face higher risk. This is also why B12 supplements can sometimes work better than food sources for people with absorption issues: high-dose supplements can be partially absorbed through passive diffusion, bypassing the intrinsic factor system entirely.
Best Food Sources
B12 is found naturally only in animal products. The richest sources include clams, beef liver, and organ meats, which can deliver dozens of micrograms in a single serving. More everyday options include salmon, tuna, beef, milk, yogurt, cheese, and eggs. A 3-ounce serving of salmon provides roughly 4 to 5 micrograms, well over the daily recommendation.
If you follow a vegan or vegetarian diet, your options are limited to fortified foods and supplements. Many plant-based milks, breakfast cereals, and nutritional yeast are fortified with B12. Nutritional yeast is a popular choice for vegans because a single tablespoon of a fortified brand can provide a full day’s worth. Without deliberate planning, though, a fully plant-based diet will almost certainly leave you short on B12 over time.
Who’s Most Likely To Be Deficient
Several groups face elevated risk. Vegans and strict vegetarians are the most obvious, since natural B12 exists only in animal foods. Older adults are also vulnerable because stomach acid production declines with age, making it harder to free B12 from food. An estimated 10 to 30 percent of adults over 50 have reduced ability to absorb food-bound B12.
People taking certain medications can also run low. Long-term use of acid-reducing drugs (commonly taken for heartburn or reflux) and the diabetes medication metformin have both been associated with lower B12 levels. Anyone who has had weight-loss surgery affecting the stomach or small intestine should be monitored regularly, since the surgery can permanently alter B12 absorption. People with autoimmune conditions affecting the stomach, particularly pernicious anemia (where the immune system attacks intrinsic factor-producing cells), need B12 supplementation for life.
Signs of Deficiency
B12 deficiency develops slowly, often over years, and early symptoms are easy to dismiss. Fatigue, weakness, and lightheadedness are common first signs. You might notice a sore, swollen tongue or mouth ulcers. Pale or slightly yellowish skin can appear as anemia progresses.
The neurological symptoms are what make B12 deficiency particularly concerning. These can include numbness or tingling in the hands and feet (peripheral neuropathy), difficulty with balance and coordination, and cognitive changes like brain fog or memory problems. In severe, prolonged deficiency, damage to the spinal cord can occur, potentially leading to difficulty walking, bowel or urinary incontinence, and even paralysis. Some people experience psychiatric symptoms, including depression, paranoia, and delusions. The neurological damage can become permanent if left untreated for too long, which is why catching a deficiency early matters.
Testing Your B12 Levels
A standard blood test measures serum B12 levels. Normal values fall between 160 and 950 picograms per milliliter (pg/mL). Levels below 160 pg/mL suggest a deficiency. However, some people develop symptoms even in the low-normal range (roughly 200 to 300 pg/mL), so a borderline result doesn’t always mean everything is fine. If results are inconclusive, a doctor may check methylmalonic acid or homocysteine levels, both of which rise when B12 is functionally low inside cells.
Supplement Forms and Absorption
B12 supplements come in several forms. Cyanocobalamin is the synthetic version most commonly used in fortified foods and inexpensive supplements. Methylcobalamin, adenosylcobalamin, and hydroxocobalamin are the three naturally occurring forms.
All four forms are absorbed into the bloodstream at similar rates, but they differ in how well the body retains them. In animal studies, methylcobalamin resulted in 13% more B12 stored in the liver compared to cyanocobalamin, and urinary excretion of cyanocobalamin was three times higher. This suggests your body holds onto the natural forms more efficiently, though both effectively raise blood levels.
One important detail about dosing: absorption efficiency drops dramatically as the dose increases. Your body absorbs about 50% of a dose under 0.5 micrograms, around 20% of a 1-microgram dose, and only about 1% of a 500-microgram dose. This is why many B12 supplements contain what looks like an absurdly high amount (1,000 or even 5,000 micrograms). At those levels, even the small percentage absorbed is enough to meet your needs.
Safety at High Doses
B12 is one of the safest vitamins to supplement. No tolerable upper intake level has been established because no toxic effects have been consistently identified, even at very high doses. Your body excretes what it doesn’t need through urine. That said, megadose supplements aren’t necessary for most people with adequate absorption. If you eat animal products regularly and don’t have an absorption issue, a standard multivitamin or a diet-focused approach is typically sufficient.