What the research says about vitamin B12 supplements

What it does in the body

The NIH Health Professional fact sheet says B12 is needed for the development, myelination and function of the central nervous system, for making healthy red blood cells, and for DNA synthesis.[1]

Forms of B12

The NIH fact sheet says B12 in supplements is usually cyanocobalamin, and other common forms are adenosylcobalamin, methylcobalamin and hydroxocobalamin. It says research has not shown that any form is better than the others.[2]

High doses and absorption

Many supplements give far more B12 than the recommended amount, but the body absorbs only a small share of it. The Health Professional fact sheet explains that B12 is absorbed with intrinsic factor, a protein made in the stomach, and absorption drops sharply once that system is full at about 1 to 2 mcg. About 50% is absorbed from doses under 1 to 2 mcg, but only about 2% of a 500 mcg dose and 1.3% of a 1,000 mcg dose.[2][1]

500 mcg dose

about 2% absorbed

1,000 mcg dose

about 1.3% absorbed

Under the tongue vs swallowed

The Health Professional fact sheet says evidence suggests no difference in how well oral and sublingual (under the tongue) B12 work.[1]

Oral B12 vs injections for people with a deficiency

A Cochrane review found 3 randomized trials (153 people) comparing B12 by mouth with injections for 3 to 4 months. At 1,000 mcg a day, oral B12 raised blood levels about as well as injections; at 2,000 mcg a day, oral B12 raised them more. The authors rated this low-quality evidence and found oral treatment cost less. No trial reported on symptoms of deficiency.[3]

Trials

3 RCTs, 153 people

Oral doses tested

1,000 or 2,000 mcg/day

Heart disease, memory and energy

The NIH fact sheet says B vitamin supplements lower homocysteine but don’t reduce the risk of heart disease or stroke, that most studies show low B12 levels don’t affect cognitive decline in older people, and that B12 doesn’t boost energy or endurance in people who already get enough from their diet.[2]

Groups the fact sheet names

The NIH fact sheet lists older adults (who may make less stomach acid), people with pernicious anemia, people who have had stomach or intestinal surgery including weight-loss surgery, people with celiac or Crohn’s disease, and vegetarians and vegans. It says adults over 50 should get most of their B12 from fortified foods or supplements.[2]

Recommended intake

The NIH fact sheet lists a Recommended Dietary Allowance of 2.4 mcg a day for adults, 2.6 mcg in pregnancy and 2.8 mcg while breastfeeding.[2]

Adults

2.4 mcg/day

Pregnancy

2.6 mcg/day

Breastfeeding

2.8 mcg/day

High doses

The NIH fact sheet says B12 has not been shown to cause harm, even at high doses.[2]

Medicines that lower B12

The fact sheet says acid-reducing medicines (such as omeprazole and lansoprazole) can lead to B12 deficiency, and metformin might reduce B12 absorption.[2]