Vitamin B12: Who Needs It, How to Take It and Which Form to Choose

Three groups of people ask me about B12 most often. People on metformin. Vegetarians. And people whose lab report says “low normal” with no explanation.

Below: how to tell whether you are short of B12, which tests to get, which form to buy and when tablets are not enough.

What vitamin B12 does in the body

B12 (cobalamin) is needed to make DNA. Without it, the bone marrow can’t produce normal red blood cells: they come out large and there are too few of them. This is megaloblastic anemia.

B12’s second job involves the nerves. It helps build myelin, the coating around nerve fibers. That is why deficiency causes numb feet, poor balance and memory problems. Sometimes the neurological symptoms come before the anemia, while the complete blood count still looks normal.

The body stores 2-5 mg of B12, mostly in the liver. That lasts for several years. So deficiency develops slowly, and someone who goes vegan may feel it 3-5 years later.

How much vitamin B12 you need a day

The European Food Safety Authority (EFSA) set an adequate intake for adults of 4 mcg a day. In pregnancy it is 4.5 mcg, while breastfeeding 5 mcg. The US recommendation for adults is 2.4 mcg.

B12 is found only in animal foods and fortified products. Absorbing it takes several steps: in the stomach, the vitamin binds to a protein called intrinsic factor, and the pair is absorbed at the end of the small intestine. One meal lets you absorb about 1.5-2 mcg this way. Around 1% of any dose also crosses the gut wall passively, without intrinsic factor. That is how 1000 mcg tablets work.

Figures are approximate, from USDA FoodData Central. The “amount for 4 mcg” column is there to give a sense of scale; nobody eats 6 g of liver. A mixed diet with meat, fish and dairy covers the need with room to spare.

Who is at risk of B12 deficiency

Vegans and vegetarians. Vegans who don’t take supplements almost always become deficient; it is a matter of time. Lacto-ovo vegetarians get some B12 from eggs and milk, but reviews show low levels are common in this group too.

People over 50. According to the NIH, 10-30% of older adults have atrophic gastritis. The stomach makes less acid, and B12 separates poorly from food proteins. Tablets and fortified foods are still absorbed normally.

People taking metformin. In the HOME trial, 390 patients with diabetes took metformin or placebo for over 4 years. B12 levels in the metformin group were 19% lower. In the large DPPOS study, each year on metformin raised the risk of deficiency by 13%.

People who take heartburn medicines for years. A study in JAMA linked 2 or more years of proton pump inhibitors (omeprazole, pantoprazole and others) with a 65% higher risk of B12 deficiency. We have a separate article on how to safely stop proton pump inhibitors.

People with stomach and bowel conditions. Autoimmune gastritis (pernicious anemia), Crohn’s disease, celiac disease, stomach surgery including bariatric procedures, removal of the last part of the small intestine.

People who inhale nitrous oxide. “Laughing gas” from balloons inactivates B12. Neurologists see young people with numbness and walking problems after regular nitrous oxide use.

B12 deficiency symptoms and tests

Typical signs: tiredness, shortness of breath, pale skin, numbness and tingling in the feet and fingers, an unsteady walk, a red smooth tongue, cracks at the corners of the mouth, worse memory and mood. None of them is specific. A diagnosis needs a blood test.

Total B12. The first test. The 2024 NICE guideline uses these cut-offs: below 180 ng/L confirms deficiency, 180-350 ng/L is a gray zone, above 350 ng/L makes deficiency unlikely. Many labs report results in pg/mL, which is the same unit: 1 pg/mL equals 1 ng/L.

Active B12 (holotranscobalamin). Shows more precisely how much B12 is available to cells. Below 25 pmol/L is deficiency, 25-70 pmol/L is a gray zone, above 70 pmol/L makes deficiency unlikely.

Homocysteine and methylmalonic acid. These are ordered when the result falls in the gray zone. Both rise in B12 deficiency.

A few practical points. If you already take a B12 supplement, the test will read high, so tell your doctor. B12 levels drop naturally in pregnancy, and results need careful interpretation. And if a blood test shows anemia, the doctor usually checks B12, folate and iron with ferritin together.

A note on folic acid. High doses can correct the anemia of B12 deficiency while nerve damage keeps progressing. So if B12 deficiency is suspected, don’t start high-dose folic acid on your own.

Forms of vitamin B12: cyanocobalamin, methylcobalamin and others

At the pharmacy you will see several names. All of them are vitamin B12, and the body turns each one into the two working forms: methylcobalamin and adenosylcobalamin.

The “cyano” in cyanocobalamin worries people. The amount of cyanide in a tablet is tiny, and the body clears it easily. The 2024 NICE guideline advises choosing a supplement with cyanocobalamin, methylcobalamin or adenosylcobalamin and does not prefer any of them.

One more detail for Ukraine. Under Ministry of Health Order No. 203, a dietary supplement may contain no more than 100 mcg of B12 per daily serving. So 500-1000 mcg B12 products used to treat deficiency can no longer be sold as supplements under the new rules. They are registered as medicines.

B12 tablets or injections

A 2018 Cochrane review compared 1000-2000 mcg tablets a day with injections and found no difference in how blood B12 levels recovered. The evidence is limited (3 randomized trials, 153 participants), but the result fits the passive absorption mechanism.

NICE splits it this way:

Lifelong injections for autoimmune gastritis, or after total removal of the stomach or the last part of the small intestine.
Injections or tablets, at the doctor’s discretion, for deficiency caused by medicines (metformin, proton pump inhibitors) or nitrous oxide.
Tablets for diet-related deficiency. Injections are chosen here if the condition is getting worse quickly or the person is unlikely to take tablets regularly.

If malabsorption is suspected, the tablet dose is at least 1 mg (1000 mcg) a day. For pregnant and breastfeeding women with deficiency, NICE also suggests considering 1 mg a day.

People usually start to feel better within 2 weeks, but full recovery can take up to 3 months. Neurological symptoms improve most slowly, and if treatment started late, some may stay.

How to take vitamin B12

For prevention, vegans and people who eat very few animal foods usually need 10-50 mcg a day or 2000 mcg once a week. Both work: a small daily dose uses intrinsic factor, and a large weekly dose relies on passive absorption.

For treatment, the doctor chooses the dose and form based on test results and the cause of deficiency. A repeat test is usually done after 3 months, or after 1 month in pregnancy.

Timing doesn’t matter. B12 is water-soluble, so you can take it before or after food, in the morning or at night. It is easy to combine with other supplements such as omega-3; they don’t interfere with each other.

If you take metformin, ask your doctor about checking B12, especially after several years of treatment or if your feet feel numb. Diabetic neuropathy and B12 deficiency cause similar symptoms and are easy to confuse.

About the skin. As a dermatologist, I’ll add this: high-dose B12 injections sometimes cause an acne-like rash on the face, back and chest. It clears after the dose is stopped or reduced. Tablets at preventive doses usually don’t cause it.

Safety. EFSA has not set an upper limit for B12 because no toxicity was found even at high doses. The excess leaves in urine. Still, a high dose without a reason does nothing for how you feel.

Questions and answers

Conclusions

An adult needs about 4 mcg of vitamin B12 a day. If your diet includes meat, fish, eggs or dairy, you usually don’t need a supplement.

Vegans always need B12. People over 50 and those who have taken metformin or proton pump inhibitors for years should have their B12 checked from time to time.

Cyanocobalamin and methylcobalamin are equally effective. 1000 mcg tablets help most people with deficiency. Injections are needed for autoimmune gastritis, after stomach or bowel surgery and for severe neurological symptoms.

About the Author

Nikolay Bondarenko is a Ukrainian dermatologist and expert in skin diseases.   A graduate of Ukraine's leading medical university, he specializes in diagnosing and treating dermatological issues.   As the author of numerous expert articles, he actively contributes to the development of dermatology and shares his knowledge with the medical community.