Vitamin b12 toxicity uptodate

Vitamin b12 toxicity uptodate DEFAULT

How I treat cobalamin (vitamin B12) deficiency

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Vitamin B12 deficiency

Summarytoggle arrow icon

Vitamin B12 (cobalamin) plays an essential role in enzymatic reactions responsible for red blood cell (RBC) formation and proper myelination of the nervous system. Thus, a deficiency of vitamin B12 may lead to megaloblastic anemia and a wide range of neurological disturbances. Deficiency may be caused by malabsorption, malnutrition, or increased demand. The most common underlying cause of vitamin B12 deficiency is pernicious anemia, an autoimmune disorder characterized by the absence of intrinsic factor (IF). IF is a protein that is crucial for vitamin B12 absorption. Patients present with signs of anemia (e.g., fatigue) and/or neurological manifestations such as paresthesia, spasticity, ataxia, and neuropsychiatric disorders. After detecting low serum vitamin B12 levels, the diagnostic approach consists of identifying the underlying cause by measuring autoantibodies and possibly conducting a Schilling test. When testing patients with suspected vitamin B12 deficiency, it is important to remember that folate deficiency also causes megaloblastic anemia. However, only vitamin B12 deficiency can be accompanied by neuropathy and exhibits elevated levels of methylmalonic acid (MMA). Treatment consists of parenteral supplementation; depending on the underlying cause, long-term supplementation may be needed.

Etiologytoggle arrow icon

Pathophysiologytoggle arrow icon

Dysfunctional biochemical reactions

Folate deficiency also leads to low levels of THF, resulting in megaloblastic anemia.

Pernicious anemia

Clinical featurestoggle arrow icon

  • Signs of anemia (e.g., fatigue, pallor)[3]
  • Mild scleral icterus and/or jaundice
  • Neurological disturbances are generally symmetrical
    • Peripheral neuropathy: tingling, numbness, pins-and-needles sensation, coldness (especially in the lower extremities)
    • Subacute combined degeneration of spinal cord: symmetricaldemyelination of the spinal cord tracts occurs in vitamin B12 deficiency due to insufficient vitamin B12-dependent fatty acid synthesis and production/maintenance of myelin
    • Neuropsychiatric disease; (e.g., reversible dementia, depression, paranoia) [3]
    • Worsening vision
    • Autonomic dysfunction: impotence and incontinence
  • Glossitis

The Spinocerebellar tracts, lateralCorticospinal tracts, and Dorsal columns are affected in Subacute Combined Degeneration.

Always consider vitamin B12 deficiency when evaluating patients with dementia.

Diagnosticstoggle arrow icon

Hematological findings


Differential diagnosestoggle arrow icon

Differential diagnoses of vitamin B12 deficiency

Differential diagnosis of vitamin B12, B9, and B1 deficiencies
Vitamin B12 deficiencyVitamin B9 deficiencyVitamin B1 deficiency
Motor signs
Sensory signs
  • Sensory deficit and paresthesias in distal parts of extremities ("socks and gloves" distribution)
  • Sensory deficit and paresthesias in distal parts of extremities ("socks and gloves" distribution)
Other neurological features
Neuropsychiatric signs
  • Confusion
  • Cognitive deficits
  • Potentially reversible (neurological damage may be permanent)

Starting folate treatment before excluding vitamin B12 deficiency may correct anemia, but it can worsen neuropathy!

In contrast to vitamin B12 deficiency, folic acid deficiency is generally not associated with neurological symptoms.

Other causes of macrocytic anemia

Other causes of neuropathy

The differential diagnoses listed here are not exhaustive.

Treatmenttoggle arrow icon


Referencestoggle arrow icon

  1. Vitamin B 12.,-dependency,-and-toxicity/vitamin-b-12. Updated: September 1, 2016. Accessed: December 21, 2016.
  2. Scalabrino G. Cobalamin (vitamin B(12)) in subacute combined degeneration and beyond: traditional interpretations and novel theories.. Exp Neurol. 2005; 192 (2): p.463-79. doi: 10.1016/j.expneurol.2004.12.020 . | Open in Read by QxMD
  3. Johnson LE. Vitamin B 12. undefined. 2016 .
  4. Mazokopakis EE. The old Schilling test as a necessary criterion at present for the diagnosis of food-cobalamin malabsorption (FCM) syndrome. Hell J Nucl Med. 2012; 15 (3): p.262-263.
  5. Ramphul K, Mejias SG. Schilling Test. StatPearls. 2020 .
  6. Guidelines for the Investigation & Management of vitamin B12 deficiency. Updated: January 3, 2017. Accessed: January 3, 2017.
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How Much Vitamin B12 Is Too Much?

Vitamin B12 is a water-soluble nutrient that plays many critical roles in your body.

Some people think that taking high doses of B12 — rather than the recommended intake — is best for their health.

This practice has led many to wonder how much of this vitamin is too much.

This article examines the health benefits, as well as potential risks of taking megadoses of B12.

Benefits of Supplementing With Vitamin B12

There’s no question that vitamin B12 is essential for health.

This nutrient is responsible for numerous functions in your body, including red blood cell formation, energy production, DNA formation and nerve maintenance ().

Though B12 is found in many foods, such as meat, poultry, seafood, eggs, dairy products and fortified cereals, many people don’t get enough of this important vitamin.

Health conditions such as inflammatory bowel disease (IBD), certain medications, genetic mutations, age and dietary restrictions can all contribute to an increased need for B12.

Vitamin B12 deficiency can lead to serious complications such as nerve damage, anemia and fatigue, which is why those at risk should add a high-quality B12 supplement to their diet ().

While people who consume adequate amounts of B12-rich foods and are able to properly absorb and utilize this nutrient don’t necessarily need to supplement, taking extra B12 has been linked to some health benefits.

For example, studies show that supplemental B12 may benefit people without a deficiency in the following ways:

  • Improved mood: One study found that supplementing healthy men with a B-complex vitamin containing a high dose of B12 improved ratings of stress and enhanced performance on cognitive tests ().
  • Reduced symptoms of anxiety and depression: Treatment with a supplement containing a high dose of B12 for 60 days significantly improved depression and anxiety symptoms in adults when compared to a placebo ().

Although B12 supplements are commonly taken to boost energy levels, there is currently no evidence suggesting that more B12 increases energy in people with adequate levels of this vitamin.

However, B12 supplements will most likely increase energy levels in those who are deficient, as this nutrient plays an important role in converting food into energy.


B12 is an important nutrient that is essential for red blood cell formation, DNA synthesis and many other vital processes. Supplements may help boost mood and reduce symptoms of depression in those who are not deficient in this vitamin.

Is Taking High Doses of B12 Helpful or Harmful?

Since B12 is a water-soluble vitamin, it’s generally considered safe, even at high doses.

No Tolerable Upper Intake Level (UL) has been established for B12, due to its low level of toxicity. UL refers to the maximum daily dose of a vitamin unlikely to cause adverse side effects in the general population.

This threshold has not been set for B12 because your body excretes whatever it doesn’t use through your urine.

However, supplementing with excessively high levels of B12 has been linked to some negative side effects.

Several studies have shown that megadoses of the vitamin can lead to outbreaks of acne and rosacea, a skin condition that causes redness and pus-filled bumps on the face.

Yet, it should be noted that most of these studies focused on high-dose injections rather than oral supplements (, 6, ).

There is also some evidence suggesting that high doses of B12 may lead to negative health outcomes in those with diabetes or kidney disease.

One study found that people with diabetic nephropathy (loss of kidney function due to diabetes) experienced a more rapid decline in kidney function when supplemented with high-dose B vitamins, including 1 mg per day of B12.

What’s more, the participants receiving the high-dose B vitamins had a greater risk of heart attack, stroke and death, compared to those receiving a placebo ().

Another study in pregnant women showed that extremely high B12 levels due to vitamin supplements increased the risk of autism spectrum disorder in their offspring ().

Though there is evidence that supplementing with B12 may cause negative health outcomes, studies have demonstrated that daily oral supplements of up to 2 mg (2,000 mcg) are safe and effective in treating B12 deficiency ().

For reference, the recommended daily intake (RDI) of vitamin B12 is 2.4 mcg for both men and women, though pregnant and breastfeeding women have a higher need ().


Although there is some evidence that very high doses of B12 may cause adverse health effects in certain populations, megadoses of this vitamin are commonly used to safely and effectively treat B12 deficiency.

How Much B12 Should You Take?

For healthy individuals who are not at risk for B12 deficiency, eating a well-rounded, healthy diet should provide all the B12 their body needs.

Food sources of this vitamin include eggs, red meat, poultry, seafood, milk, yogurt, fortified cereals, nutritional yeast and fortified non-dairy milks.

However, individuals on medications that impact B12 absorption, pregnant or breastfeeding women, vegans and anyone with a condition that negatively impacts the absorption of or increases the need for B12, should consider taking a supplement.

Additionally, evidence from population studies suggests that B12 deficiency in older adults is common, which is why it’s recommended that adults over 50 take supplements ().

While megadoses of up to 2,000 mcg are considered safe in treating B12 deficiency, it’s always best to avoid excessive amounts of any vitamin, especially when it’s not needed.

Though daily high doses of B12 are unlikely to cause harm in most people, extremely high doses should be avoided unless prescribed by a healthcare professional.

If you think you may be deficient in B12, speak with your doctor, who can recommend an appropriate treatment based on your level of deficiency.

While no UL has been set for B12, your body’s ability to absorb the vitamin depends on how much it actually needs.

For example, it’s estimated that only 10 mcg of a 500-mcg B12 supplement is actually absorbed in people without a deficiency ().

For this reason, taking high doses of B12 does not benefit people without an increased need.


Although supplemental B12 is required for people with an increased need for this vitamin, it’s unnecessary for those without a deficiency to take high doses.

The Bottom Line

B12 is an important nutrient that is popularly used as a nutritional supplement, even by those without a B12 deficiency.

Though doses of up to 2,000 mcg of vitamin B12 are considered safe, it’s best to speak to a doctor to find out whether taking a supplement is necessary.

Most people can fill their B12 needs through a healthy diet. Some, such as older adults or those with certain dietary restrictions, should supplement.

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