What blocks absorption of B12? Common causes and effective solutions

October 15, 2025 •

4 min read

According to research, as many as 20% of individuals over the age of 60 in the United States and the United Kingdom are deficient in vitamin B12, often due to issues with malabsorption. This essential nutrient requires a complex process to be properly absorbed, and numerous factors can block absorption of B12, leading to deficiency.

Quick Summary

Several factors can hinder B12 absorption, including autoimmune diseases like pernicious anemia, stomach acid deficiencies, intestinal disorders, and common medications like metformin and antacids.

Key Points

Pernicious Anemia: An autoimmune condition that prevents the production of intrinsic factor, a protein crucial for B12 absorption.
Reduced Stomach Acid: Medications like PPIs and H2 blockers, as well as atrophic gastritis, reduce the stomach acid needed to release B12 from food.
Intestinal Damage: Disorders such as Crohn's and celiac disease can damage the part of the small intestine where B12 is absorbed.
Specific Medications: The diabetes drug Metformin, long-term antibiotics, and gout medication colchicine can all inhibit B12 absorption.
Dietary Restrictions: Vegan and vegetarian diets increase the risk of B12 deficiency because natural sources are primarily animal-based.
Alcohol and Nitrous Oxide: Both excessive alcohol use and recreational nitrous oxide abuse can impair B12 absorption and utilization.

Understanding the B12 Absorption Process

To understand why B12 absorption can fail, it helps to know how it's supposed to work. The process is a complex journey through the digestive system. First, in the stomach, hydrochloric acid and enzymes separate vitamin B12 from the protein it is bound to in food. Next, the free B12 binds to a protein called intrinsic factor, which is produced by the stomach's parietal cells. This new complex then travels to the small intestine, specifically the terminal ileum, where it is absorbed into the bloodstream. A breakdown at any point in this pathway can lead to malabsorption.

Medical Conditions Affecting B12 Absorption

Pernicious Anemia

Pernicious anemia is the most well-known cause of B12 malabsorption. It is an autoimmune disorder where the body's immune system attacks and destroys the parietal cells in the stomach that produce intrinsic factor. Without intrinsic factor, dietary B12 cannot be properly absorbed, regardless of how much is consumed. This condition most commonly affects people over 60 and those of Northern European descent.

Gastrointestinal and Pancreatic Disorders

Various diseases of the digestive tract can impair B12 absorption, including:

Crohn's Disease: Chronic inflammation of the small intestine can damage the terminal ileum, where B12 absorption takes place.
Celiac Disease: An autoimmune reaction to gluten that damages the lining of the small intestine, interfering with nutrient absorption.
Small Intestinal Bacterial Overgrowth (SIBO): An excess of bacteria in the small intestine can consume B12, leaving less for the body to absorb.
Pancreatic Insufficiency: A lack of pancreatic enzymes can prevent the release of B12 from haptocorrin, another protein it binds to early in the digestive process.

Atrophic Gastritis

This condition is characterized by chronic inflammation of the stomach lining, which leads to reduced production of both hydrochloric acid and intrinsic factor. It is a common cause of B12 deficiency in older adults, who are more likely to have lower stomach acid levels naturally.

Medications That Block B12 Absorption

Long-term use of certain medications is a significant and often overlooked cause of B12 malabsorption.

Proton Pump Inhibitors (PPIs) and H2 Blockers: Medications like omeprazole (Prilosec), lansoprazole (Prevacid), and ranitidine (Zantac) reduce stomach acid, inhibiting the release of B12 from food.
Metformin: This diabetes drug can interfere with the absorption of B12 in the intestines, particularly with high doses and long-term use.
Antibiotics: Long-term use can alter the gut microbiome, potentially affecting B12 levels.
Colchicine: A medication for gout that can also interfere with B12 absorption.

Dietary and Lifestyle Factors

Vegan and Vegetarian Diets

Since vitamin B12 is found naturally only in animal-based products like meat, fish, eggs, and dairy, individuals following strict vegan or vegetarian diets are at high risk for deficiency unless they consume fortified foods or supplements.

Excessive Alcohol Consumption

Chronic alcohol abuse can damage the lining of the stomach and intestine, impairing the production of stomach acid and intrinsic factor necessary for proper B12 absorption.

Nitrous Oxide Abuse

Recreational use of nitrous oxide (laughing gas) can inactivate B12, preventing its utilization by the body.

Comparison of B12 Malabsorption Causes


Category	Specific Cause	Mechanism	Who is at Risk?
Autoimmune	Pernicious Anemia	Immune system attacks intrinsic factor-producing stomach cells.	Individuals of Northern European or African descent, those over 60.
Gastrointestinal	Atrophic Gastritis	Chronic inflammation reduces stomach acid and intrinsic factor.	Older adults.
Gastrointestinal	Crohn's or Celiac Disease	Damage to the lining of the small intestine.	Individuals with diagnosed intestinal disorders.
Medication	PPIs and H2 Blockers	Reduced stomach acid, which is needed to free B12 from food.	Individuals on long-term acid-reducing medication.
Medication	Metformin	Interferes with B12 absorption in the small intestine.	Individuals with type 2 diabetes.
Lifestyle	Veganism	Lack of animal-based food sources naturally containing B12.	Individuals on strict vegan diets.
Lifestyle	Excessive Alcohol	Damages the stomach lining and digestive system.	Individuals with chronic alcohol use disorder.

Strategies to Overcome Absorption Barriers

For those with diagnosed malabsorption issues, simply eating more B12-rich foods is often not enough. Effective strategies depend on the underlying cause:

B12 Injections: For severe malabsorption, particularly from pernicious anemia, regular B12 injections bypass the digestive system entirely to ensure the vitamin reaches the bloodstream.
High-Dose Oral Supplements: The body can absorb a small amount of B12 (around 1%) passively without intrinsic factor. For individuals with mild to moderate malabsorption, high-dose oral supplements may be an effective alternative to injections.
Addressing Underlying Issues: Treating conditions like H. pylori infection or SIBO can improve B12 absorption over time.
Dietary and Lifestyle Changes: For those on plant-based diets, consistent use of fortified foods and high-quality supplements is crucial. Limiting or eliminating excessive alcohol consumption can also support better digestive health and B12 uptake.

Conclusion

Understanding what blocks absorption of B12 is the first step toward preventing or treating a deficiency. Whether the cause is an autoimmune condition, a digestive disorder, a long-term medication, or a dietary choice, effective solutions are available. It is crucial to consult a healthcare provider for an accurate diagnosis, especially if you experience symptoms like fatigue, tingling, or neurological changes. A personalized treatment plan can help restore B12 levels and prevent long-term complications. For those seeking further information on diagnosis and treatment, resources like the National Institutes of Health Office of Dietary Supplements offer authoritative guidance.

Disclaimer: This article is for informational purposes only and does not constitute medical advice. Consult with a healthcare professional for a personalized assessment and treatment plan.

Frequently Asked Questions

Yes, long-term use of antacids like proton pump inhibitors (PPIs) and H2 blockers reduces the amount of stomach acid. This stomach acid is necessary to separate vitamin B12 from the food protein it is bound to, thus impairing absorption.

Yes, long-term use of metformin, a common medication for type 2 diabetes, can reduce the absorption of vitamin B12 in the small intestine. Regular monitoring of B12 levels is often recommended for individuals on this medication.

Pernicious anemia is an autoimmune disease where the body mistakenly attacks its own stomach cells, preventing them from producing intrinsic factor. Intrinsic factor is essential for the gut to absorb vitamin B12.

Inflammatory bowel diseases like Crohn's disease can damage the lining of the small intestine, specifically the terminal ileum, where B12 is absorbed. In celiac disease, the gluten-triggered immune response also damages the gut lining, causing general malabsorption.

B12 is naturally found almost exclusively in animal products. While some plant-based foods are fortified, strict dietary practices without supplementation or fortified foods can lead to deficiency due to insufficient intake.

Yes, excessive alcohol consumption can damage the stomach lining over time. This damage reduces the production of stomach acid and other factors needed to properly absorb B12.

For some causes of malabsorption, high-dose oral supplements may be effective because the body can absorb a small amount of B12 passively without intrinsic factor. However, for severe cases like pernicious anemia, injections are typically required. It is important to consult a healthcare provider for the right course of treatment.

As people age, they are more prone to developing atrophic gastritis, a condition that reduces stomach acid and intrinsic factor production. This makes it harder for older adults to absorb B12 from food.