By Rachel Nall, MSN, CRNA

Reviewed by Alana Biggers, MD, MPH

Some people may worry that supplements containing large amounts of vitamin B-12 could be harmful. However, although some supplements — such as vitamin A or D — can be harmful if a person takes them in excess, vitamin B-12 seems safe in large doses. Vitamin B-12 is vital for human life due to its role in making red blood cells and supporting the function of the brain and nerves.

If a person’s diet does not provide enough vitamin B-12, a doctor may suggest taking supplements, some of which contain more than the daily recommended dose. The body does not store vitamin B-12, so taking extra is not harmful.

This article covers the recommended daily values for vitamin B-12, research into the effects of taking too much B-12, and how vitamin B-12 might affect pregnancy.

How much vitamin B-12 is too much?

The Office of Dietary Supplements (ODS) do not provide an upper limit for vitamin B-12. This is because people usually tolerate vitamin B-12 well with no unwanted side effects.

According to an article in the journal American Family Physician, researchers have found that supplements containing as much as 1,000 times the recommended daily allowance (RDA) are safe.

The body does not absorb vitamin B-12 well. In fact, research suggests that the body absorbs just 9.7 micrograms (mcg) of a 500 mcg dose, which is just 2%.

According to the ODS, the RDA of vitamin B-12 by age is as follows:

• 0–6 months: 0.4 mcg
• 7–12 months: 0.5 mcg
• 1–3 years: 0.9 mcg
• 4–8 years: 1.2 mcg
• 9–13 years: 1.8 mcg
• 14 years and older: 2.4 mcg

During pregnancy, the RDA of vitamin B-12 is 2.6 mcg. A woman needs 2.8 mcg during lactation.

Can too much vitamin B-12 cause side effects?

Specialists are not currently aware of any side effects associated with taking higher-than-recommended levels of vitamin B-12.

Some researchers have even studied the potential side effects of taking high doses of vitamin B-12 for up to 5 years at a time. However, no studies have yet found any adverse symptoms or patterns of harm.

Specialists divide vitamins into two categories: water soluble and fat soluble. Water soluble vitamins dissolve in water and exit the body via the urine. Vitamin B-12 is a water soluble vitamin, and this is one of the reasons why higher vitamin B-12 levels are not toxic.

According to the Linus Pauling Institute at Oregon State University in Corvallis, there is currently no evidence to suggest any adverse effects from:

• taking oral vitamin B-12 in doses as high as 2 milligrams (mg)
• taking vitamin B-12 via injection in doses as high as 1 mg

Doctors tend to prescribe these higher dosages to treat conditions related to very low vitamin B-12 levels, such as pernicious anemia.

Although vitamin B-12 appears, for the most part, to be safe — even at higher dosages — more is not necessarily better.

There is no need to take very high doses of vitamin B-12. There is no data to suggest that very high supplementation levels are any more effective at boosting a person’s immune system or bodily functions if the person does not have a vitamin B-12 deficiency.

Can people have high vitamin B-12 blood levels?

Healthcare professionals have not identified an upper limit blood level of vitamin B-12. The body will typically excrete excess vitamin B-12 that a person gets from the diet or via supplements.

The body does not absorb vitamin B-12 supplements very effectively. This means that even if a person takes very high amounts of vitamin B-12, their body still does not absorb a significant amount.

In addition, taking some medications may impair the body’s ability to absorb vitamin B-12. Examples of these medications include:

• chloramphenicol (Chloromycetin), which is an antibiotic
• H2 receptor antagonists
• metformin
• proton pump inhibitors

Pregnancy and vitamin B-12

Vitamin B-12 supplements are safe to take during pregnancy.

In fact, the ODS recommend that women who follow strict vegetarian or vegan diets should take vitamin B-12 supplements, as deficiency can severely affect the fetus.

For example, low vitamin B-12 levels in pregnant women can lead to an increased risk of neural tube defects, preterm birth, preeclampsia, and pregnancy loss, according to a review paper in The American Journal of Clinical Nutrition.

The authors conducted a literature review on how frequently pregnant women were vitamin B-12 deficient. They found that, on average, around 25% of pregnant women were deficient in vitamin B-12 across all trimesters.

Research in the journal American Family Physician has found that the children of vitamin B-12 deficient women who breastfeed are more likely to experience failure to thrive, developmental delays, anemia, and weakness.

Taking vitamin supplements or having vitamin B-12 injections may be protective against side effects that doctors associate with deficiency.

Low vitamin B-12 levels

Many people do not get enough vitamin B-12 from their diet.

Vitamin B-12 is naturally present in many animal products, including meat, fish, dairy, and eggs. Plant based sources of vitamin B-12 include fortified milks and cereals, nutritional yeast, and vegan spreads such as Marmite.

However, doctors tend to recommend taking vitamin B-12 supplements if a person struggles to meet their daily requirements. People can find vitamin B-12 supplements in drugstores or choose from several types online.

Low vitamin B-12 levels can give rise to a number of health concerns, including:

• appetite loss
• balance problems
• constipation
• fatigue
• sores on the mouth or tongue
• weakness

Summary

Although there is currently no evidence to suggest any adverse side effects from excess vitamin B-12 supplementation, there is no need to take excessively high amounts of the vitamin.

Most healthcare providers do not recommend exceeding the amount available in prenatal or daily multivitamins, unless the person has a condition that causes them to be vitamin B-12 deficient.

If a person is concerned about their vitamin B-12 levels, they should talk to their doctor.