What is anemia caused by lack of B12?

November 16, 2025 •

4 min read

Vitamin B12 deficiency anemia, also known as megaloblastic anemia, is a condition where the body produces abnormally large, dysfunctional red blood cells due to insufficient vitamin B12. This deficiency compromises the blood's ability to transport oxygen efficiently, leading to widespread fatigue and other potentially serious symptoms.

Quick Summary

A lack of vitamin B12 leads to a specific type of anemia characterized by the production of abnormally large red blood cells. The underlying cause can range from poor dietary intake to impaired absorption due to conditions like pernicious anemia or certain surgeries. This impairs oxygen delivery, causing fatigue and neurological problems.

Key Points

Megaloblastic Anemia: B12 deficiency leads to the production of abnormally large, dysfunctional red blood cells, impairing oxygen delivery.
Pernicious Anemia: A common cause is this autoimmune disorder, where the body cannot absorb B12 due to a lack of intrinsic factor.
Neurological Risks: The deficiency can cause irreversible nerve damage, leading to tingling, memory issues, and balance problems if left untreated.
Diverse Causes: Causes range from dietary insufficiency in vegans to malabsorption issues caused by surgery, chronic diseases, or medication.
Treatment is Lifelong: Treatment often involves regular B12 injections or high-dose oral supplements, and many people need lifelong management, especially with pernicious anemia.
Diagnosis is Crucial: Tests for B12 levels, MMA, and antibodies are used for diagnosis, which is key for starting early and effective treatment.

Understanding B12 Deficiency Anemia

Anemia caused by a lack of B12 is a type of megaloblastic anemia, where red blood cells become oversized and oval-shaped rather than small and round. This abnormality prevents them from functioning correctly, reducing their lifespan and diminishing their ability to carry oxygen. While dietary insufficiency can be a cause, particularly in vegans or vegetarians, the problem is more commonly rooted in an inability to absorb the vitamin properly. This is often due to a condition called pernicious anemia, an autoimmune disease that prevents the absorption of B12 in the stomach.

How Vitamin B12 and Intrinsic Factor Work

To be absorbed, dietary vitamin B12 must bind to a protein called intrinsic factor, which is produced by cells in the stomach lining. This complex is then absorbed in the small intestine. When the immune system attacks these intrinsic factor-producing cells, as is the case with pernicious anemia, or if the cells are removed surgically, the body cannot absorb the vitamin, regardless of dietary intake.

Key Causes of B12 Deficiency Anemia

Several factors can lead to B12 deficiency and subsequent anemia. They can be broadly categorized into dietary and malabsorption issues.

Dietary Causes

Vegan or Vegetarian Diets: Since B12 is primarily found in animal products, individuals following strict vegan or vegetarian diets are at higher risk if they do not supplement or consume fortified foods.
Alcoholism: Excessive alcohol use can interfere with the body's ability to absorb and store vitamin B12.

Malabsorption Issues

Pernicious Anemia: An autoimmune condition where the body's immune system attacks and destroys the stomach cells that produce intrinsic factor.
Gastric Surgery: Procedures that remove part of the stomach or small intestine, such as gastric bypass, can decrease intrinsic factor production or reduce the surface area for absorption.
Intestinal Diseases: Conditions like Crohn's disease, celiac disease, and bacterial overgrowth (SIBO) can impair B12 absorption.
Certain Medications: Long-term use of medications like metformin (for diabetes) or some acid-reducing drugs can affect B12 levels.

Recognizing the Symptoms

Symptoms of B12 deficiency anemia often develop slowly over time and can be mild at first, making them easy to overlook. The severity increases as the deficiency worsens. Signs can include:

Fatigue and Weakness: Persistent tiredness and lack of energy are common due to the reduced oxygen-carrying capacity of the blood.
Neurological Symptoms: B12 is crucial for a healthy nervous system, so a deficiency can cause: numbness or tingling in the hands and feet, memory loss, confusion, and poor balance.
Gastrointestinal Issues: Loss of appetite, nausea, weight loss, and a swollen or sore tongue (glossitis) can occur.
Psychological Changes: Irritability, depression, and other mood or behavioral changes are possible.
Pale or Jaundiced Skin: A yellowish tinge to the skin or eyes can result from the breakdown of abnormal red blood cells.

Diagnosis and Treatment

Diagnosis typically involves a physical exam and review of medical history, followed by blood tests. These tests may include a complete blood count (CBC), serum B12 levels, and specific tests like methylmalonic acid (MMA) and homocysteine levels, which confirm the deficiency. For suspected pernicious anemia, tests for intrinsic factor or gastric parietal cell antibodies may be used.

Treatment aims to restore normal B12 levels. The approach depends on the underlying cause. For absorption issues like pernicious anemia, oral supplements are often ineffective, and vitamin B12 injections are required. These injections may be given frequently at first and then spaced out to a monthly schedule. Some people with absorption issues may be able to transition to high-dose oral B12 supplements over time, but many require lifelong treatment. Dietary counseling is important for those with poor intake. Early treatment is crucial to prevent irreversible nerve damage.

Comparison of B12 vs. Iron Deficiency Anemia


Feature	B12 Deficiency Anemia (Megaloblastic)	Iron Deficiency Anemia (Microcytic)
Cause	Inadequate B12, often due to malabsorption (e.g., pernicious anemia).	Inadequate iron, often due to poor intake or blood loss.
Red Blood Cell Size	Abnormally large (macrocytic).	Abnormally small (microcytic).
Appearance	Pale skin, sometimes with jaundice.	Pale skin.
Neurological Symptoms	Common; tingling, memory issues, balance problems.	Uncommon.
Fatigue	Common and often severe.	Common.
Associated Conditions	Autoimmune diseases, gastric surgery, intestinal disorders.	Heavy menstruation, ulcers, cancer.
Treatment	B12 injections or high-dose oral supplements.	Iron supplements and dietary changes.

Conclusion

Anemia caused by a lack of B12 is a manageable condition, but its potential for causing permanent nerve damage makes early diagnosis and treatment vital. Understanding the primary causes, particularly the distinction between dietary deficiency and malabsorption issues like pernicious anemia, is key to determining the correct therapeutic approach. With proper diagnosis and consistent treatment, most individuals can successfully control their symptoms and prevent long-term complications. If you experience persistent fatigue, numbness, or other related symptoms, consult a healthcare provider for proper evaluation. For more information on anemia and its different types, an excellent resource is the Mayo Clinic's detailed overview.

Frequently Asked Questions

The most common cause of anemia from B12 deficiency is pernicious anemia, an autoimmune condition where the body cannot produce the intrinsic factor needed for B12 absorption.

Early signs of B12 deficiency often include persistent fatigue, weakness, a feeling of lightheadedness, and a pale or yellowish tinge to the skin.

Yes, a severe or long-term B12 deficiency can lead to significant nerve damage, which may result in tingling or numbness in the hands and feet, difficulty with balance, and memory problems.

While inadequate intake is a cause, especially for those on strict vegan diets, malabsorption issues are more common. This means the body can't absorb B12 even if enough is consumed.

It is diagnosed through a combination of a physical examination, review of symptoms, and blood tests to check vitamin B12 levels, methylmalonic acid (MMA), and homocysteine levels.

Treatment depends on the cause. For malabsorption, injections of vitamin B12 are typically prescribed. Some people with less severe deficiencies or different underlying causes may use high-dose oral supplements.

Yes, low levels of vitamin B12 are linked to neurological symptoms like confusion, short-term memory loss, and difficulty concentrating, particularly in severe or long-term cases.