Medically Reviewed by: Jared Meacham, PhD, RD, CSCS and Patricia Weiser, PharmD

Low vitamin B12 levels are usually caused by a deficiency in the diet or problems absorbing the nutrient. But, in some cases, they may point to a more serious health condition. Here’s what you should know about the causes, symptoms, and treatment of a vitamin B12 deficiency.

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Feeling foggy and fatigued lately? While there are plenty of potential causes, a vitamin B12 deficiency could be one of them. This essential nutrient has a big influence on our energy levels and nervous system function, so when levels dip too low, it can initiate a cascade of physical and mental symptoms.

A B12 deficiency is often caused by diet or absorption problems. But in some rare cases, it can point to more serious conditions. We’ll take a look at what causes low B12, the symptoms to watch for, how a deficiency is diagnosed, and what you can do to treat it.

What does vitamin B12 do?

Vitamin B12 (also called cobalamin, as it contains the mineral cobalt) is a water-soluble vitamin that can be found in animal foods, including red meat and dairy. B12 binds to the proteins in those foods and has to be released from those proteins before it can be absorbed in the body. 

This isn’t the same for B12 supplements or foods that have B12 added to them. “The B12 found in fortified foods and supplements is unbound, or free, and therefore is able to be absorbed even by people with absorption conditions,” explains Bailey Franklyn, an Ontario-based registered dietitian.

The release process begins when food is chewed and binds to a protein called R-factor that’s in saliva. When food gets to the pancreas, enzymes release it from the R-factor, and B12 then clings to a protein called intrinsic factor. This new combo allows B12 to be absorbed into the small intestines and used by the body.

Vitamin B12 is important for these functions:

• Red blood cell production: B12 helps the body make healthy red blood cells, preventing anemia and the fatigue that frequently accompanies it. Red blood cells carry oxygen through the body via hemoglobin. 
• Energy metabolism: B12 helps break down carbohydrates into glucose and metabolize fats, which provide energy at the cellular level.
• Nervous system function: B12 maintains the myelin sheath—the protective covering around nerves—supporting brain health and coordination.‍
• DNA synthesis: B12 plays a role in creating and repairing DNA.

What causes low vitamin B12?

Levels of B12 can dip too low for various reasons, though diet is a big one. Here are some of the main causes of  vitamin B12 deficiency. 

Dietary insufficiency

Vitamin B12 is found almost exclusively in animal products, such as meat, dairy, and eggs. Some plant-based foods, however, are fortified with B12, including nutritional yeast, breakfast cereals, and plant milks. People who follow a vegan or strict vegetarian diet are at a higher risk of deficiency unless they take supplements or consume plenty of B12-fortified foods.

Malabsorption issues

Even if you consume plenty of B12 through food, your body may not absorb it properly. Conditions that interfere with stomach acid or intrinsic factor (IF)—a protein made by the stomach that helps absorb B12—can lead to deficiency. Some examples include:

• Pernicious anemia: Anemia is a lack of sufficient red blood cells. This type of anemia is caused by poor B12 absorption. This occurs when the body attacks IF or the cells in the stomach that make it. B12 then is unable to bind to IF, which is needed in order to be absorbed in the small intestine for use throughout the body. Pernicious anemia is the most common cause of B12 deficiency in the UK, according to the National Health Services. Pernicious anemia also increases the risk of developing stomach cancer or multiple myeloma, according to the Canadian Cancer Society.  
• Atrophic gastritis: This chronic inflammation of the stomach lining can reduce stomach acid secretion, and also impact B12 absorption. Atrophic gastritis can be caused by an autoimmune condition or by a bacterial stomach infection called Helicobacter pylori (H. pylori). Some studies estimate 10% to 30% of older Canadian adults have this condition.

Gastrointestinal disorders and surgeries

Some GI-related conditions and surgeries may affect B12 absorption:

• Crohn’s disease: A chronic inflammatory disease, Crohn’s affects the small and large intestine — particularly the distal ileum, which is where B12 is absorbed. 
• Celiac disease: The autoimmune disorder damages the small intestine, where B12 is absorbed for use in the body. 
• Gastric bypass and ileal resection surgeries: B12 deficiencies are common after these surgeries, in part because the procedures remove or bypass B12 absorption sites in the gut.

Medications

Some commonly prescribed drugs can reduce B12 levels:

• Metformin: Metformin treats type 2 diabetes and can cause B12 absorption problems, especially when taken long term. While it’s not totally understood yet how this happens, some researchers theorize it could have something to do with the medication’s impact on intrinsic factor. 
• Proton pump inhibitors (PPIs) and H2-receptor blockers: Vitamin B12 needs stomach acid for proper absorption. Ongoing use of medications that treat acid reflux or gastroesophageal reflux disorder (GERD) can interfere with this process and cause low B12 levels.
  

Aging

Vitamin B12 deficiency is more common in older people, affecting about 1 in 20 people aged 65 to 74 and 1 in 10 people aged 75 and older. As we age, stomach acid production naturally declines, which can make it harder to extract B12 from food. Many older adults don’t get enough B12 even with a seemingly normal diet. Plus, older adults are more likely to have pernicious anemic or atrophic gastritis, which also affect B12 levels. 

Although it’s a common condition, B12 deficiency in older adults often goes undiagnosed, as it’s not routinely tested. 

Symptoms of vitamin B12 deficiency

The signs of B12 deficiency can be subtle at first, but if left untreated, they may become more severe or even irreversible. Symptoms may also take a while to emerge. “Our body is able to store some B12, therefore, it may take years for symptoms to present,” Franklyn says.

Here are the common symptoms of a vitamin B12 deficiency:

• Fatigue and weakness
• Brain fog and memory problems
• Numbness or tingling in hands and feet (called peripheral neuropathy)
• Shortness of breath
• Mood changes, such as depression or anxiety

Can vitamin B12 deficiency be a sign of cancer?

Although rare, vitamin B12 deficiency can sometimes point to an underlying cancer, especially cancers that affect the gastrointestinal tract. 

Research on this connection is mixed. While some studies point to low levels of B12 being associated with certain cancers, other research shows that higher B12 levels are associated with more advanced stages of certain cancers. 

“Vitamin B12 deficiency can indicate various underlying issues, including cancer, but it’s important to recognize that cancer is not the most common cause,” says Tanya ter Keurs, a nurse practitioner and NiaHealth’s co-founder and clinical director. 

Certain cancers can interfere with the body’s ability to absorb or use vitamin B12, ter Keurs says. “However, it’s important to note that many other more common conditions, including taking certain medications, are more likely to be the culprit: for example, a lack of dietary intake, taking medications for acid reflux or diabetes or an autoimmune condition,” she adds.

How Cancer May Interfere with B12 Levels

Research isn’t conclusive on vitamin B12 and cancer. There are some ways that cancer may affect B12 levels, but if you're a meat-eater, you have about 10 years of B12 in storage in your body. An acute cancer is very unlikely to affect this.

• Stomach cancer can destroy parietal cells that produce intrinsic factor, so B12 won’t be absorbed sufficiently. Most gastric cancers are associated with a Helicobacter pylori bacterial infection—the same infection that often leads to atrophic gastritis and subsequent B12 absorption problems. 
• Colorectal cancer may block, damage, or disrupt absorption areas. If surgery is required to remove a tumor or part of the colon, that may also impede absorption. 
• Pancreatic cancer surgery may reduce digestive enzyme production, which can impair nutrient absorption, so enzyme replacements are typically prescribed to prevent this. But research has not found a clear association between vitamin B12 levels and pancreatic cancer.
• Liver cancer could affect how B12 is stored and metabolized, as excess vitamin B12 is stored in the liver. When liver function is compromised, the liver’s ability to store and release B12 may be impaired, ter Keurs explains. As a result, B12 levels can either be abnormally high, due to the release of B12 from damaged liver cells, or low, due to reduced storage and release.

Still, low B12 levels alone almost never means you have cancer. Most people with B12 deficiency do not have cancer, but if persistent and/or coupled with other concerning symptoms,  unexplained cases should be evaluated by a physician.

“Low B12 may indicate a more severe condition, however, it’s unlikely that it would be the first, or only symptom,” Franklyn says. “If your B12 is low without any explanation—you eat foods with vitamin B12 or you haven’t been diagnosed with a condition that causes low B12—then it would be worth investigating further with your healthcare provider.”

Can high B12 levels be a warning sign?

Not necessarily. There is no conclusive evidence that high B12 levels are intertwined with cancer. However, research from the UK found high B12 levels were associated with a higher risk of cancer diagnosis within one year—though the mechanism remains unclear. If you have high B12 levels, don't panic, and rely on your primary care clinician to help understand what's happening.

B12 is a marker, not a cause, and elevated levels should be considered alongside other clinical signs and tests.

In certain cases, some studies have found links between high serum B12 levels and the following conditions:

• Certain cancers, including lung and liver cancer
• Liver dysfunction,  as some research has found high levels of B12 in the bloodstream can indicate damaged liver tissue.  
• Blood disorders, such as leukemia or polycythemia vera

Overall, more research is needed to understand whether high (or low) B12 levels affect cancer risk. Until then, there's no reason to panic.

Vitamin B12 testing and diagnosis

If you or your physician suspect a B12 deficiency, you’ll go through various diagnostic tests to check your levels.

• Serum B12 test: This is typically the initial test of B12. Levels below 200 to 250 pg/mL are considered deficient, though symptoms can appear even at slightly higher levels.
• Methylmalonic acid (MMA) and homocysteine: These biomarkers rise when B12 levels are low, often before serum B12 drops, helping to detect deficiencies.
• Intrinsic factor antibodies and parietal cell antibodies: These tests are used to diagnose pernicious anemia or atrophic gastritis. The presence of these antibodies can indicate B12 absorption problems. 
• Additional gut health testing: In cases of a persistent or unexplained B12 deficiency, your doctor may recommend endoscopy, colonoscopy, or tests for celiac or Crohn’s disease.

“Gut health testing is beneficial because B12 absorption occurs in the small intestine, and conditions that affect gut function—such as bacterial overgrowth, malabsorption disorders, or inflammatory diseases—can lead to B12 deficiency,” ter Keurs says.

Gut health testing can identify these conditions, say ter Keurs:

• Gut flora imbalances: An overgrowth of certain bacteria might interfere with the absorption of B12.
• Gastrointestinal issues: Conditions like celiac disease or Crohn's disease can impair B12 absorption in the gut.
• Digestive enzyme deficiencies: These could affect B12 release from food, further hindering absorption.
  

“Understanding gut health can help pinpoint the cause of B12 deficiency and guide appropriate treatment, such as addressing bacterial overgrowth or managing an underlying gastrointestinal disease,” ter Keurs says.

Vitamin B 12 treatment and monitoring

The good news? Vitamin B12 deficiency is usually easy to treat, especially once the cause is identified. Treatment options can include:

Supplements

• Oral B12: Effective for most people, even those with mild malabsorption
• Sublingual B12: Dissolves under the tongue, dominantly orally with some absorption passing into the bloodsteam
• Injectable B12: Used in severe cases or where absorption is significantly impaired

Treatment of the root cause

This might mean treating celiac disease, adjusting medications, or managing autoimmune conditions like pernicious anemia.

Additional  monitoring

Follow-up testing is key to ensure your levels return to and stay within a healthy range. This may include repeat serum B12, MMA, or complete blood count (CBC).

When to see a doctor

Early treatment can prevent long-term complications, and, in rare cases, uncover more serious underlying conditions. If you have any of these symptoms, ter Keurs recommends speaking to your physician or scheduling a B12 blood test. 

• Persistent fatigue or weakness
• Numbness or tingling, especially in the hands and feet
• Memory loss, brain fog, concentration difficulties, or mood changes 
• Pale or jaundiced skin
• Shortness of breath or dizziness

In some cases, you may not show any signs of abnormal B12 levels. A consultation with a physician is still a good idea if:

• Your B12 levels are persistently low or high without explanation
• You have a family history of autoimmune conditions or GI cancers
• You are vegan, over age 60, or on long-term medications that may interfere with B12
  

“If symptoms persist, especially if they worsen, seeking medical attention is important  to determine the cause and start appropriate treatment,” ter Keurs says. “Early intervention can help prevent irreversible damage, particularly to the nervous system.”

The bottom line

Vitamin B12 deficiency is very common, but still a complex condition. While it’s often easily treatable, persistent or unexplained abnormal B12 levels should get medical attention. They may point to underlying conditions such as malabsorption syndromes, autoimmune issues, or, in rare cases, certain cancers.

If you’re experiencing symptoms of low B12 or want to ensure your levels are optimal, speak with a NiaHealth clinician about vitamin B12 testing, plus comprehensive nutrient and gut health testing. 

Key takeaways

• A vitamin B12 deficiency is usually caused by not getting enough of the vitamin in the diet or by a condition that makes it hard for the body to absorb B12 properly.
• Because vitamin B12 is found only in animal-based foods, people following vegan or vegetarian diets have a higher risk of a B12 deficiency.
• In some cases, low B12 levels can indicate a more serious health condition, including gastrointestinal cancers. 
• Common symptoms of a B12 deficiency include fatigue, brain fog, numbness or tingling in the hands or feet, and mood changes. 
• Blood tests can determine if you have a B12 deficiency and help your healthcare provider the best treatment options.

Frequently asked questions (FAQ's)

Does cancer cause low vitamin B12?

Some cancers, particularly those affecting the stomach or intestines, may interfere with B12 absorption. However, low B12 alone is rarely the first sign of cancer and often does not indicate cancer at all.

What are the two main causes of B12 deficiency?

The most common causes are dietary insufficiency (especially in vegans) and malabsorption (often due to pernicious anemia or inherited GI disorders).

What are the worst symptoms of B12 deficiency?

An untreated B12 deficiency can lead to irreversible nerve damage, cognitive decline, and severe anemia.

What is the fastest way to fix B12 deficiency?

Weekly injections work fastest, especially in severe cases of malabsorption. For most people, high-dose oral or sublingual supplements are effective.

What are the warning signs of vitamin B12 deficiency?

Watch for fatigue, tingling in the hands or feet, brain fog, memory issues, pale skin, and mood changes.

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