What Illness Causes Low B12? Exploring the Root Causes

October 19, 2025 •

2 min read

Statistics indicate that vitamin B12 deficiency is a common issue, particularly among older adults, often stemming from complex absorption problems rather than just diet. If you're concerned about your levels, understanding what illness causes low B12 is the first step toward effective diagnosis and management.

Quick Summary

Various illnesses and conditions cause low B12, including autoimmune pernicious anemia, chronic gastritis, inflammatory bowel diseases like Crohn's, and certain medications and surgical procedures.

Key Points

Pernicious Anemia: An autoimmune disorder is a leading cause of low B12 due to the body's inability to produce intrinsic factor, a protein necessary for absorption.
Gastrointestinal Disorders: Conditions like Crohn's and Celiac disease can damage the intestinal lining, disrupting the absorption of vitamin B12.
Medication Side Effects: Long-term use of certain drugs, particularly metformin and stomach acid inhibitors, can interfere with the body's ability to absorb B12.
Surgical Impact: Weight-loss surgeries like gastric bypass and procedures that remove parts of the stomach or small intestine significantly impair B12 absorption, requiring lifelong supplementation.
Beyond Diet: While poor dietary intake is a factor, especially for vegans, the most common causes of clinical deficiency are malabsorption issues related to underlying medical conditions.
Neurological Risks: Untreated low B12 can lead to serious and potentially irreversible neurological damage, emphasizing the importance of early diagnosis and treatment.

The Crucial Role of Vitamin B12

Vitamin B12, or cobalamin, is essential for DNA production, red blood cell formation, and nervous system health. As the body doesn't produce it, B12 must come from animal products, fortified foods, or supplements. Deficiency can cause fatigue, neurological issues, and anemia.

Primary Medical Conditions Causing Low B12

Malabsorption is the most common reason for clinical B12 deficiency, even more than dietary inadequacy. Several conditions interfere with B12 absorption.

Pernicious Anemia

Pernicious anemia is an autoimmune disease where the body attacks stomach cells that produce intrinsic factor (IF), necessary for B12 absorption. Without IF, B12 cannot be absorbed effectively. This condition can be hereditary and is more prevalent in people of Northern European or African descent.

Gastrointestinal Diseases

Disorders causing inflammation or damage in the small intestine, where B12 is absorbed, are major causes of deficiency. Conditions affecting this area include:

Crohn's disease: Inflammation in the digestive tract, particularly the terminal ileum, impairs B12 absorption.
Celiac disease: Immune reaction to gluten damages the small intestine, leading to malabsorption and potentially low B12.
Chronic gastritis: Stomach lining inflammation reduces stomach acid needed to release food-bound B12, potentially leading to atrophic gastritis and pernicious anemia.
Small Intestinal Bacterial Overgrowth (SIBO): Excess small intestine bacteria consume B12.
Parasitic infections: Fish tapeworms can absorb B12 intended for the host.

Other Autoimmune Disorders

Pernicious anemia often co-occurs with other autoimmune conditions. Those with Type 1 diabetes, autoimmune thyroid disease, or vitiligo have an increased risk of pernicious anemia and low B12.

Medications and Surgeries that Impact B12 Absorption

Certain medical interventions can directly interfere with B12.

Medication-Induced Deficiency

Long-term use of specific medications can cause low B12 by interfering with absorption. These include:

Metformin: This diabetes drug can decrease B12 absorption with long-term use.
PPIs and H2 blockers: Acid-reducing drugs can hinder B12 absorption as stomach acid is needed to release B12 from food.
Certain antiseizure medications: Some may also reduce B12 absorption.

Surgical Procedures

Operations altering the digestive system, such as gastric bypass or removal of part of the stomach or terminal ileum, significantly reduce the ability to absorb B12. {Link: MSD Manuals https://www.msdmanuals.com/home/disorders-of-nutrition/vitamins/vitamin-b12-deficiency} provides more details on B12 deficiency.

Conclusion: The Path to Diagnosis and Treatment

Identifying what illness causes low B12 is crucial for treatment and preventing complications.

For more information, the {Link: NHLBI, NIH https://www.nhlbi.nih.gov/health/anemia/vitamin-b12-deficiency-anemia} provides resources on Vitamin B12 Deficiency Anemia.

Frequently Asked Questions

The most common illness causing low B12 due to malabsorption is pernicious anemia, an autoimmune disorder that prevents the stomach from producing intrinsic factor, a protein vital for B12 absorption.

Yes, Crohn's disease can cause B12 deficiency, especially when it affects the terminal ileum of the small intestine, which is the primary site for B12 absorption.

Yes, chronic gastritis or an inflamed stomach lining can decrease stomach acid production, hindering the release of B12 from food and leading to a deficiency over time.

Long-term use of metformin for diabetes, as well as proton pump inhibitors and H2 blockers for heartburn, are known to interfere with B12 absorption.

Yes, gastric bypass surgery and other procedures that modify the stomach or small intestine can significantly impair B12 absorption, often requiring lifelong supplementation.

Other risk factors include advanced age, following a strict vegan diet without supplementation, and chronic alcohol abuse, which all affect B12 levels.

Diagnosis typically begins with a blood test to check B12 levels, followed by tests for intrinsic factor antibodies or gastric endoscopy to determine if pernicious anemia or a digestive issue is the cause.