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Vitamin B12 Toxicity - Causes, Symptoms, Diagnosis, and Treatment

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Excessive vitamin B12 intake can cause rare toxicity. Read further to learn more.

Medically reviewed by

Dr. Muhammad Zohaib Siddiq

Published At September 14, 2023
Reviewed AtNovember 8, 2023


Vitamin B12, an essential water-soluble vitamin, plays a critical role in numerous physiological functions. While its deficiency can lead to severe health issues, vitamin B12 toxicity is a rare condition. This article aims to provide an overview of vitamin B12 toxicity, its causes, symptoms, diagnosis, treatment, and prevention. Understanding these aspects is crucial to maintain a balanced intake of this essential nutrient and ensuring optimal health.

What Is Vitamin B12?

Vitamin B12 is also called cobalamin. It is a water-soluble vitamin that belongs to the B-vitamin family. It is an essential nutrient required for various crucial physiological processes in the human body. Unlike some other vitamins, the body cannot produce vitamin B12 on its own, so it must get it from dietary sources or supplements.

Key functions of vitamin B12 include:

  • Red Blood Cell Formation: Vitamin B12 is important and helps produce and mature red blood cells in the bone marrow. Without sufficient vitamin B12, red blood cells can become abnormally large and inefficient, leading to a condition called megaloblastic anemia.

  • Nervous System Support: Vitamin B12 is involved in the maintenance of the nervous system. It plays a role in the synthesis of myelin, a protective sheath around nerves that aids in proper nerve transmission. A vitamin B12 deficiency can cause neurological symptoms such as tingling, numbness, and difficulty walking.

  • DNA Synthesis: Vitamin B12 is crucial for producing DNA, the genetic material in the cells. It helps with the replication and division of cells, which is essential for growth and tissue repair.

  • Energy Metabolism: Vitamin B12 is used in metabolizing carbohydrates, fats, and proteins, converting them into energy for the body to use.

What Are the Sources of Vitamin B12?

Vitamin B12 is naturally present in certain animal-derived foods, making it more abundant in diets that include animal products. Good dietary sources of vitamin B12 include:

  • Meat (such as beef, pork, and lamb).

  • Poultry (chicken, turkey).

  • Fish (salmon, trout, tuna).

  • Dairy products (milk, cheese, yogurt).

  • Eggs.

  • Fortified foods, for example, fortified breakfast cereals and plant-based milk alternatives, can also be sources of vitamin B12 for individuals following a vegetarian or vegan diet.

What Is Vitamin B12 Toxicity?

Vitamin B12 toxicity, also known as vitamin B12 overdose, is a rare condition that arises when there is an excessive accumulation of vitamin B12 in the body. Vitamin B12, or cobalamin, is a water-soluble vitamin important for various critical physiological functions. It plays a crucial role in red blood cell formation, nervous system health, DNA synthesis, and energy metabolism. Vitamin B12 is essential for overall health, but excessive intake can have adverse effects. Understanding the risks of vitamin B12 toxicity is crucial.

What Are the Causes of Vitamin B12 Toxicity?

The causes of vitamin B12 toxicity are mainly related to excessive intake or impaired excretion of the vitamin. Here are the primary causes:

  • High-Dose Supplementation: One of the common causes of vitamin B12 toxicity is taking extremely high doses of vitamin B12 supplements. While vitamin B12 is generally considered safe and has a low risk of toxicity when obtained from a normal diet or standard supplement doses, excessive intake can lead to an accumulation of the vitamin in the body. The recommended intake for adults is around 2.4 micrograms (mcg) per day, and excessive amounts, typically above 1,000 mcg or more, can lead to toxicity.

  • Pernicious Anemia Treatment: Pernicious anemia is a condition where the body has difficulty absorbing vitamin B12 from the digestive system. In some cases, vitamin B12 injections are used to treat this condition. While these injections are essential for individuals with pernicious anemia to address their deficiency, careful dosage monitoring is crucial to avoid vitamin B12 toxicity during treatment.

  • Impaired Excretion: The human body efficiently excretes excess vitamin B12 through urine. However, certain medical conditions, such as liver disease or kidney disorders, can impair the body's ability to process and excrete vitamin B12. As a result, the vitamin can accumulate in the body, leading to potential toxicity.

What Are the Signs and Symptoms of Vitamin B12 Toxicity?

The following are the symptoms of Vitamin B12 Toxicity:

  • Itching or skin rash.

  • Gastrointestinal problems, like diarrhea, nausea, and vomiting.

  • Vision changes, like blurred vision or difficulty focusing.

  • Nervous system issues, such as numbness, tingling, or a feeling of pins and needles in the extremities.

  • Difficulty walking, loss of balance, or problems with coordination.

  • Fatigue and weakness.

  • Shortness of breath.

  • Paleness of the skin (pallor).

  • Confusion or cognitive impairment (in severe cases).

  • Dizziness or lightheadedness.

How to Diagnose Vitamin B12 Toxicity?

Diagnosing vitamin B12 toxicity involves a combination of clinical evaluation, medical history review, and laboratory tests.

  • Medical History: A healthcare professional will take a detailed medical history, including information about the patient's diet, vitamin B12 supplementation, and any existing medical conditions.

  • Symptom Assessment: The presence of symptoms associated with vitamin B12 toxicity, such as skin reactions, gastrointestinal disturbances, vision changes, nervous system issues, and fatigue, will be evaluated.

  • Physical Examination: A complete physical examination will be conducted to check for specific signs of vitamin B12 toxicity, such as skin rash or neurological deficits.

  • Blood Tests: Blood tests are crucial in the diagnosis. Vitamin B12 levels in the blood will be measured to determine if they are significantly elevated. Elevated blood levels of vitamin B12 can indicate toxicity.

  • Complete Blood Count (CBC): A CBC may be performed to check for abnormalities in red blood cells and white blood cells. Vitamin B12 toxicity can affect red blood cell formation and lead to specific changes in blood cell parameters.

  • Liver and Kidney Function Tests: Tests to assess liver and kidney function may be conducted to determine if any underlying health conditions contribute to vitamin B12 toxicity.

  • Neurological Examination: If neurological symptoms are present, a neurological examination may be performed to assess motor skills, sensation, and coordination.

  • Other Tests: Additional tests may be conducted to understand other potential causes of the symptoms and to assess any organ damage associated with vitamin B12 toxicity.

How to Treat Vitamin B12 Toxicity?

The primary treatment for vitamin B12 toxicity is to discontinue the intake of excessive vitamin B12 supplements or injections. Once the source of excessive vitamin B12 is identified, it should be discontinued immediately to prevent further accumulation. The body can effectively eliminate the excess vitamin B12 through urine once supplementation is stopped. During this period, supportive care may be provided to manage any symptoms that the patient may be experiencing, such as gastrointestinal disturbances or skin reactions.

In cases where vitamin B12 toxicity has led to neurological symptoms, specialized neurological care may be required to manage these symptoms and monitor the patient's neurological status. Patients should also be educated about the importance of adhering to recommended vitamin B12 intake guidelines and avoiding self-medication with high-dose vitamin B12 supplements. Follow-up appointments with healthcare providers are crucial to monitor vitamin B12 levels and ensure they return to normal. Overall, treatment focuses on allowing the body to restore its natural balance of vitamin B12 and manage any associated symptoms effectively.

How to Prevent Vitamin B12 Toxicity?

Preventing vitamin B12 toxicity is relatively straightforward and can be achieved by following these measures:

  • Adhering to Recommended Daily Intake: Ensure that vitamin B12 supplementation stays within the recommended daily intake guidelines, typically around 2.4 micrograms (mcg) for adults.

  • Consult with Healthcare Professionals: Seek advice from healthcare professionals before starting any vitamin B12 supplementation, especially if considering high doses.

  • Balanced Diet: Most individuals can meet their vitamin B12 needs through a balanced diet that includes foods rich in nutrients, like meat, fish, dairy products, and fortified cereals.


While vitamin B12 is essential for maintaining optimal health, toxicity is a rare occurrence. Understanding the causes, symptoms, diagnosis, treatment, and prevention of vitamin B12 toxicity is vital to promote safe supplementation and ensure overall well-being. By following recommended guidelines and seeking guidance from healthcare professionals, individuals can harness the benefits of vitamin B12 while minimizing the risks associated with excessive intake.

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Dr. Muhammad Zohaib Siddiq
Dr. Muhammad Zohaib Siddiq



vitamin b12 toxicity
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