Which disease occurs due to deficiency of B12?

November 14, 2025 •

4 min read

According to the NHS, pernicious anemia is the most common cause of vitamin B12 deficiency in the UK, an autoimmune condition where the body cannot absorb this vital nutrient. This inability to absorb vitamin B12 results in a condition called megaloblastic anemia and can lead to serious neurological damage if left untreated.

Quick Summary

A lack of vitamin B12 most notably causes pernicious anemia, an autoimmune disease that impairs absorption and leads to megaloblastic anemia. This deficiency disrupts DNA synthesis and red blood cell formation, also causing neurological damage affecting the spinal cord and peripheral nerves.

Key Points

Pernicious Anemia: The most common disease caused by B12 deficiency, an autoimmune condition preventing absorption by destroying intrinsic factor.
Megaloblastic Anemia: A condition where a lack of B12 or folate leads to abnormally large and immature red blood cells, resulting in fatigue and weakness.
Neurological Damage: A severe and prolonged B12 deficiency can damage nerve cells and the spinal cord, leading to tingling, numbness, memory problems, and difficulty walking.
Cognitive and Psychological Issues: Low B12 levels are linked to depression, irritability, confusion, and in advanced stages, dementia and paranoia.
Dietary and Absorption Issues: While diet can be a factor, most chronic deficiencies are caused by malabsorption due to autoimmune conditions, gastrointestinal surgery, or certain medications.
Diagnosis and Treatment: Diagnosis is done via blood tests, and treatment involves oral supplements or, for malabsorption issues like pernicious anemia, regular B12 injections.

Understanding the Core Disease: Pernicious Anemia

While many people may associate vitamin B12 deficiency with poor diet, the primary cause of a sustained deficiency is often a condition called pernicious anemia. This is an autoimmune disorder where the body's immune system attacks the parietal cells in the stomach that produce intrinsic factor. Intrinsic factor is a crucial protein that binds with vitamin B12, allowing it to be absorbed in the small intestine. Without this binding agent, vitamin B12 simply passes through the digestive tract unabsorbed.

The Impact of B12 Deficiency on the Body

When the body cannot absorb enough B12, it directly affects several critical functions. One of the most prominent impacts is on the production of red blood cells. Vitamin B12 is essential for DNA synthesis, which is required for the formation and maturation of these cells in the bone marrow.

Blood Cell Production: Insufficient B12 leads to the formation of abnormally large, immature, and fragile red blood cells known as megaloblasts. These cells are less effective at carrying oxygen throughout the body, resulting in megaloblastic anemia. This can cause symptoms such as chronic fatigue, weakness, shortness of breath, and pale skin.
Neurological Function: Vitamin B12 is vital for the health of the nervous system. It helps in the maintenance of the myelin sheath, a protective layer that surrounds nerve fibers. A B12 deficiency can cause this sheath to break down, leading to nerve damage and a wide range of neurological symptoms. These can start as mild issues and progress to more severe, sometimes irreversible, damage.
Psychological and Cognitive Effects: The impact of B12 deficiency on the brain can manifest in psychological and cognitive problems, including memory loss, confusion, depression, and irritability. In advanced cases, this can even lead to more serious conditions like dementia and paranoia.

Comparison of B12 Deficiency-Related Anemias


Feature	Megaloblastic Anemia	Pernicious Anemia
Underlying Cause	Deficiency of B12 and/or folate.	An autoimmune attack on intrinsic factor, specifically blocking B12 absorption.
Root of B12 Problem	Can be due to diet, malabsorption, or other issues.	Primarily a malabsorption issue caused by the immune system.
Primary Symptom	Large, ineffective red blood cells.	The cause of megaloblastic anemia in many cases.
Associated Risks	Nerve damage can occur even without severe anemia.	Increased risk of stomach cancer due to chronic inflammation.
Required Treatment	B12 and/or folate supplementation.	Lifelong B12 supplementation, often via injections.

Causes Beyond Pernicious Anemia

While pernicious anemia is a significant cause, other factors can lead to a vitamin B12 deficiency. These include:

Dietary Insufficiency: Vegetarians and vegans who do not consume animal products (the primary source of B12) are at risk if they do not take supplements or consume fortified foods.
Gastrointestinal Surgery: Procedures like gastric bypass can remove or alter the part of the stomach or small intestine needed for B12 absorption.
Medical Conditions: Chronic diseases such as Crohn's disease, celiac disease, and certain pancreatic disorders can interfere with nutrient absorption. H. pylori infection has also been linked to B12 malabsorption.
Medications: Some drugs, including metformin for diabetes and long-term use of proton pump inhibitors for acid reflux, can reduce B12 levels.
Age: Older adults are more susceptible to B12 deficiency, often due to decreased stomach acid, which is needed to separate B12 from food proteins.

Diagnosing and Treating B12 Deficiency

Diagnosing a B12 deficiency typically involves a physical examination and blood tests. A complete blood count (CBC) may show macrocytosis (abnormally large red blood cells), and specific vitamin B12 levels will be measured. In some cases, levels of methylmalonic acid and homocysteine may also be tested, as these increase when B12 is lacking.

Treatment depends on the cause and severity. For dietary deficiencies, oral supplements or a change in diet may be sufficient. In cases of pernicious anemia or severe malabsorption, vitamin B12 injections are necessary to bypass the digestive tract. This is often a lifelong treatment plan. Early diagnosis and treatment are crucial to prevent permanent neurological damage.

Conclusion

In summary, the most prominent disease stemming from a deficiency of B12 is megaloblastic anemia, which in many instances is a result of the autoimmune condition pernicious anemia. However, other factors like diet and medication can also play a role. The impact of a B12 deficiency goes beyond anemia, affecting the nervous system and cognitive function. Timely diagnosis and lifelong management, especially for conditions like pernicious anemia, are vital to prevent irreversible neurological complications. It is always important to consult a healthcare provider for a proper diagnosis and treatment plan if you suspect a deficiency. For further reading on related conditions, consider consulting the MSD Manuals website.

Frequently Asked Questions

Yes, a severe and prolonged B12 deficiency can cause serious nerve damage, or neuropathy. This can result in tingling or numbness in the hands and feet, muscle weakness, and impaired physical coordination.

A doctor can test for B12 deficiency through a blood test that measures the levels of vitamin B12. Additional tests for methylmalonic acid (MMA) and homocysteine may also be used to confirm a diagnosis, especially in borderline cases.

The most common cause of vitamin B12 deficiency is a malabsorption problem, not a lack of it in the diet. Pernicious anemia, an autoimmune disorder, is a leading cause where the body cannot absorb B12 properly due to a lack of intrinsic factor.

Yes, a B12 deficiency can lead to a variety of psychological and cognitive problems. These can include irritability, depression, memory loss, and confusion. In very severe cases, it can contribute to dementia and paranoia.

Pernicious anemia is a lifelong autoimmune condition. Individuals with this disease need to receive ongoing vitamin B12 supplementation, often through injections, for the rest of their lives to manage their levels and prevent complications.

Yes, because vitamin B12 is found primarily in animal products, people following strict vegan or vegetarian diets are at risk for B12 deficiency. They must supplement with fortified foods or take oral supplements to ensure adequate intake.

Megaloblastic anemia is a type of anemia characterized by the bone marrow producing abnormally large, immature red blood cells. It is caused by deficiencies of either vitamin B12 or folate, both of which are necessary for DNA synthesis.