Vitamin B12

What it is

Vitamin B12 is a water-soluble vitamin, found naturally in some foods, added to others, and sold as a supplement and prescription medicine. Water-soluble vitamins dissolve in water, and leftover amounts leave the body in urine.

Sources: NIH Office of Dietary Supplements: Vitamin B12 - Health Professional Fact Sheet ↗ · MedlinePlus (NIH): Vitamin B12: MedlinePlus Medical Encyclopedia ↗

How it's thought to work

• Vitamin B12 is required for nervous system development and function, healthy red blood cell formation, and DNA synthesis.
• Food B12 is attached to protein and must be released before the body absorbs it.
• The body absorbs B12 by combining it with intrinsic factor, a protein made by the stomach.

Sources: NIH Office of Dietary Supplements: Vitamin B12 - Health Professional Fact Sheet ↗ · NIH Office of Dietary Supplements: Vitamin B12 - Consumer ↗

What people use it for

A prescription shot of B12 is usually used to treat vitamin B12 deficiency. Manufacturers often promote B12 supplements for energy, athletic performance, and endurance.

Source: NIH Office of Dietary Supplements: Vitamin B12 - Consumer ↗

What the research has found so far

• The fact sheet says B12 does not give energy or endurance benefits in people who get enough from their diet.
• Research on cancer is mixed: some studies link high B12 levels to higher cancer risk, others link low levels, others find no link.
• A review of studies found B12 supplements alone or with other B vitamins do not prevent heart attacks or reduce death rates in people at risk of or with heart disease.
• B12 supplements lower homocysteine, but research shows they do not reduce the risk of cardiovascular disease or stroke.

Sources: NIH Office of Dietary Supplements: Vitamin B12 - Consumer ↗ · NIH Office of Dietary Supplements: Vitamin B12 - Health Professional Fact Sheet ↗

On Vitamin B12 and aging, PubMed lists 520 trials in people, 1,676 studies that watched people over time, 48 animal and lab studies. See the counts and searches

Side effects and risks

• The consumer fact sheet says vitamin B12 has not been shown to cause any harm, even at high doses.
• No upper intake limit was set for vitamin B12 because it has low potential for toxicity.
• In one study of men, using at least 55 mcg a day of supplemental B12 for about 10 years was linked to a 40% higher lung cancer risk.

Sources: NIH Office of Dietary Supplements: Vitamin B12 - Consumer ↗ · NIH Office of Dietary Supplements: Vitamin B12 - Health Professional Fact Sheet ↗

Who should be careful, and drug interactions

• Groups more likely to be deficient include older adults, people with pernicious anemia or gut disorders, and those who have had gut surgery.
• Vegetarians and vegans have a higher risk of deficiency because natural B12 comes only from animal foods.
• Metformin might reduce B12 absorption and lower blood B12 levels.

Sources: NIH Office of Dietary Supplements: Vitamin B12 - Health Professional Fact Sheet ↗ · NIH Office of Dietary Supplements: Vitamin B12 - Consumer ↗

Amounts used in studies

What studies or labels describe, not a recommendation.

• Some supplements give much higher doses than recommended, such as 500 mcg or 1,000 mcg.
• Studies compared very high doses (1,000–2,000 mcg) of oral B12 with injections for deficiency.
• Absorption is only about 2% at 500 mcg and 1.3% at 1,000 mcg.

Sources: NIH Office of Dietary Supplements: Vitamin B12 - Consumer ↗ · NIH Office of Dietary Supplements: Vitamin B12 - Health Professional Fact Sheet ↗

Roughly what it costs

The 11 products we list that contain Vitamin B12 cost from $9.79 to $40, as read from the makers' pages. See the products

Questions to ask your doctor

• Is Vitamin B12 a sensible option for me, given my health and the medicines I take?
• Could Vitamin B12 interact with any medicine or supplement I already take?
• What side effects should I watch for, and when should I stop or call you?
• Supplements aren't checked by the FDA before sale. Is there a brand or a third-party test seal you would trust?
• How would we tell whether it is helping, and when should we review it?