What Causes Deficiency Of B12?

Vitamin B12 deficiency happens when the body doesn’t get enough B12 from food, can’t absorb what it gets, or uses more than usual. The most common cause worldwide is simply not eating enough animal-based foods, since B12 occurs naturally almost exclusively in meat, fish, eggs, and dairy. In the United States and other countries where meat is common, the leading cause is different: a problem with absorption, often because the stomach or small intestine can’t do their job properly.

That distinction matters. Two people can have the same low B12 level for completely different reasons, and the fix is not the same for both.

What Causes Deficiency Of B12?

B12 deficiency almost always traces back to one of three problems: you’re not taking enough in, your body can’t absorb it, or your body is using it up faster than it can be replaced. Absorption problems are the most common cause in people who eat a typical Western diet.

B12 absorption is a multi-step process, and a breakdown at any step can cause deficiency. Here’s how it normally works:

• Stomach acid and an enzyme called pepsin release B12 from the proteins in food.
• A protein made in the stomach called intrinsic factor binds to the freed B12.
• This B12–intrinsic factor pair travels to the end of the small intestine (the ileum), where it is absorbed into the bloodstream.

If any link in that chain fails, B12 from food cannot get in. This is why absorption disorders are so central to understanding deficiency.

How Does Not Eating Enough B12 Cause Deficiency?

B12 is found naturally in animal foods. Plants do not make it. That single fact explains why diet is such a powerful cause.

People who avoid all animal products — strict vegetarians and vegans — are at the highest risk from diet alone. The body stores a meaningful reserve of B12 in the liver, so a deficiency from diet typically develops slowly, often over years rather than weeks. That long runway is why someone who changes their diet may feel fine for a long time before symptoms appear.

Other groups who may not get enough through diet include older adults who eat poorly or have limited food variety, and people with alcohol use disorder, whose diets are often low in nutrients and whose absorption is also impaired.

It’s worth noting that some plant foods and supplements are fortified with B12. Fortified foods can help, but the amount and reliability vary by product.

Why Does Absorption Failure Cause Most B12 Deficiency?

In countries where meat and dairy are widely eaten, absorption failure — not low intake — is the main driver of deficiency. The stomach and small intestine are where things most often go wrong.

Several distinct conditions can interrupt absorption:

• Pernicious anemia. The immune system attacks the stomach cells that make intrinsic factor. Without intrinsic factor, B12 cannot be absorbed at all. This is a classic and well-documented cause.
• Atrophic gastritis. Long-term inflammation thins the stomach lining and reduces acid and intrinsic factor production. It becomes more common with age.
• Stomach or intestinal surgery. Removing part of the stomach or the ileum removes the tissue that makes intrinsic factor or absorbs B12.
• Crohn’s disease and celiac disease. These can damage the ileum or interfere with absorption.
• Bacterial overgrowth. Excess bacteria in the small intestine can consume B12 before the body absorbs it.
• Fish tapeworm infection. A parasite acquired from eating certain raw or undercooked fish can take up B12 in the gut.

Each of these blocks absorption at a different point, but the end result is the same: less B12 reaches the bloodstream.

Can Medications Cause B12 Deficiency?

Yes. Some widely used medications interfere with B12 absorption, and this is well established.

Long-term use of acid-suppressing drugs — proton pump inhibitors and histamine-2 blockers — reduces stomach acid. Less acid means less B12 is freed from food. The evidence for this effect is strongest with prolonged use and higher doses.

The diabetes medication metformin is also linked to lower B12 levels. Research suggests metformin can reduce B12 absorption, and monitoring is often recommended for people on long-term therapy. This is one of the more clinically recognized drug–nutrient interactions.

If you take either type of medication long term, that is a conversation to have with your clinician — not a reason to stop a prescribed drug on your own.

Who Is Most at Risk for B12 Deficiency?

Risk rises when intake and absorption problems overlap. Some groups carry more than one risk factor at once.

• Adults over 60, because stomach acid and intrinsic factor decline with age.
• Vegans and strict vegetarians, from diet alone.
• People with autoimmune conditions such as pernicious anemia, type 1 diabetes, or thyroid disease, which tend to cluster.
• People who have had gastric bypass or intestinal surgery.
• People with Crohn’s disease, celiac disease, or chronic gut conditions.
• Long-term users of acid reducers or metformin.
• People with alcohol use disorder.

Pregnancy and breastfeeding raise B12 needs, and deficiency in a mother can affect the infant. No clinical guidelines set a universal screening schedule for all pregnant women, so this is typically managed based on individual risk and clinical judgment.

What Symptoms and Signs Point to B12 Deficiency?

B12 deficiency affects the blood and the nervous system, and symptoms reflect both. Because the body stores B12, symptoms often build gradually and can be easy to miss early on.

Blood-related signs include fatigue, weakness, pale skin, and a specific type of anemia in which red blood cells become unusually large. This is called macrocytic anemia.

Nervous system signs are the ones that worry clinicians most, because some can become permanent if the deficiency goes untreated for too long. They include:

• Tingling or numbness in the hands and feet.
• Problems with balance and walking.
• Memory trouble, confusion, or difficulty concentrating.
• Irritability or changes in mood.

A non-obvious point: nerve symptoms can appear even when there is no anemia at all. A normal blood count does not rule out B12 deficiency, which is why clinicians look at B12 levels and related markers, not just red blood cells.

How Is B12 Deficiency Diagnosed and Treated?

Diagnosis usually starts with a blood test for vitamin B12. Because the standard test can miss some cases, clinicians often add other markers — methylmalonic acid and homocysteine — which can be elevated when B12 is low even if the basic B12 level looks borderline.

Treatment depends on the cause. For a dietary shortfall, B12 supplements or fortified foods can correct it. For absorption problems, high-dose oral B12 sometimes works, but injections are often used because they bypass the gut entirely. The right approach depends on why the deficiency happened in the first place.

What’s clear from the evidence is that treating the underlying cause matters as much as replacing the vitamin. If a medication or a gut condition is driving the deficiency, that issue needs attention too.