How do you feel when your B12 is deficient? Understanding the Signs and Symptoms

October 13, 2025 •

4 min read

Up to 20% of adults over 60 may have low or borderline vitamin B12 levels, though symptoms can begin earlier. So, how do you feel when your B12 is deficient? The reality is a wide and often confusing array of physical, neurological, and psychological symptoms that can deeply affect your quality of life.

Quick Summary

Vitamin B12 deficiency can cause profound fatigue, nerve issues like tingling and numbness, and psychological symptoms including depression and anxiety. Onset is often slow, with manifestations worsening over time and sometimes mistaken for other conditions.

Key Points

Gradual Onset: Symptoms of B12 deficiency develop slowly, over months or even years, making them easy to overlook.
Profound Fatigue: One of the most common early indicators is a deep, persistent fatigue and general weakness that doesn't resolve with rest.
Neurological Issues: B12 deficiency frequently causes tingling or numbness in the hands and feet, memory problems, and loss of balance.
Psychological Changes: Mood disturbances such as depression, anxiety, and irritability are also associated with low B12 levels.
Malabsorption is a Key Cause: Many deficiencies are caused by the body's inability to properly absorb B12 due to conditions like pernicious anemia or gastric surgery, not just a poor diet.
Treatable but Potentially Irreversible: While treatment can resolve many symptoms, delayed or untreated deficiency can lead to permanent nerve damage.
Importance of Diagnosis: Due to overlapping symptoms with other conditions, accurate diagnosis through blood tests is critical for effective treatment.

The Insidious Onset of Deficiency

Unlike many other vitamin deficiencies, the symptoms of low vitamin B12 often develop gradually over several months or even years. This slow progression can make it difficult for individuals to connect their evolving health issues to a single cause. It is not uncommon for early signs, like fatigue and mild cognitive difficulties, to be dismissed as normal aging or stress, delaying a proper diagnosis. The body stores significant amounts of B12 in the liver, which can mask the problem for a prolonged period after dietary intake or absorption issues begin.

The Physical Toll of Low B12

One of the most common and earliest signs of vitamin B12 deficiency is a profound and unrelenting fatigue. This is often accompanied by a general feeling of weakness and lethargy that doesn't improve with rest. The body relies on B12 to create red blood cells, which carry oxygen. Without enough B12, the result is often megaloblastic anemia, where red blood cells are abnormally large and unable to function effectively, leading to a lack of oxygen reaching tissues and organs.

Other physical symptoms can include:

Pale or yellowish skin: Due to the decrease in healthy red blood cells.
Sore, red tongue (glossitis) or mouth ulcers: A painful or swollen tongue can be an indicator.
Weight loss and loss of appetite: As a result of general unwellness and digestive disturbances.
Nausea, vomiting, or diarrhea: Gastrointestinal issues are also common.
Heart palpitations and shortness of breath: The heart works harder to circulate oxygen in cases of severe anemia.

Neurological and Psychological Impacts

The most serious consequences of a B12 deficiency are those affecting the nervous system. The vitamin is critical for maintaining healthy nerve cells, and a lack thereof can cause significant nerve damage if left untreated.

Brain Fog and Mood Changes

Many individuals report experiencing 'brain fog,' characterized by confusion, difficulty concentrating, and impaired memory. On the psychological front, B12 deficiency has been strongly linked to mood disturbances. As B12 is essential for synthesizing neurotransmitters like serotonin, low levels can lead to feelings of sadness, irritability, anxiety, and even depression. Some severe cases may even involve symptoms like paranoia or psychosis.

Nerve Damage and Neuropathy

A hallmark neurological symptom is peripheral neuropathy, manifesting as tingling, numbness, or a "pins and needles" sensation, particularly in the hands and feet. This can progress to difficulty with walking, balance problems (ataxia), and muscle weakness, as the nerve damage worsens. In advanced stages, permanent nerve damage can occur, making early intervention crucial.

Comparing B12 Deficiency and Folate Deficiency

Because B12 and folate work closely together in the body and deficiencies can produce similar anemia symptoms, a proper diagnosis is key. Here is a comparison:


Feature	Vitamin B12 Deficiency	Folate (B9) Deficiency
Onset of Symptoms	Gradual, over months or years, due to liver storage.	Can occur more quickly, often within weeks, as body stores are smaller.
Key Neurological Signs	Distinctive neurological symptoms common, such as tingling, numbness, memory loss, and balance issues.	Neurological symptoms are typically absent, even with severe anemia, unless there is a coexisting B12 deficiency.
Psychological Effects	Can cause depression, anxiety, irritability, and cognitive issues.	Can also cause mood disturbances like depression.
Associated with Anemia	Can cause megaloblastic anemia.	Also a cause of megaloblastic anemia.
Masking by Supplementation	Folic acid supplementation can mask a B12 deficiency by correcting the anemia, but allows neurological damage to progress.	No such masking effect with B12 supplementation.

Common Causes of B12 Deficiency

Several factors can lead to a deficiency, beyond simply not consuming enough B12-rich foods:

Dietary Insufficiency: Primarily affects vegans and strict vegetarians, as B12 is naturally found in animal products like meat, eggs, and dairy. Fortified foods or supplements are necessary for these groups.
Malabsorption Conditions: Many cases are caused by the body's inability to absorb the vitamin properly.
- Pernicious Anemia: An autoimmune condition where the body attacks a protein called intrinsic factor, which is needed for B12 absorption.
- Gastritis: Inflammation of the stomach lining can reduce hydrochloric acid needed to release B12 from food.
- Digestive Disorders: Conditions like Crohn's disease and celiac disease can impair absorption in the small intestine.
Medications: Certain drugs, including metformin for diabetes and some acid reflux medications, can interfere with B12 absorption.
Aging: As people get older, their ability to absorb B12 from food can decline.

What Happens If It Goes Untreated?

If vitamin B12 deficiency is left to progress, the consequences can be severe and potentially irreversible. The nerve damage can become permanent, leading to persistent neuropathy, vision problems, and cognitive decline. In severe, long-term cases, it can result in neurological deficits, heart problems (due to anemia), and an increased risk of gastric cancer in people with pernicious anemia. It is a condition that must be taken seriously and addressed promptly by a healthcare professional.

Conclusion: Listen to Your Body

The symptoms of vitamin B12 deficiency can be varied, gradual, and easily confused with other health issues, which is why proper diagnosis by a doctor is essential. While fatigue and weakness are common indicators, pay close attention to any strange neurological sensations, changes in mood, or cognitive difficulties. Early detection and treatment, whether through injections or oral supplements, can reverse many of the symptoms and prevent serious, long-term complications. By being aware of how you feel when your B12 is deficient, you can empower yourself to seek the necessary medical attention and restore your well-being. For more detailed information, consult the National Institutes of Health (NIH) factsheet on Vitamin B12.

Frequently Asked Questions

The earliest signs often include unexplained fatigue, weakness, and a lack of energy. These can be subtle and develop gradually over time, so they may not seem like a significant problem at first.

Yes, it can. Low B12 levels are linked to psychological issues such as depression, anxiety, irritability, and confusion because the vitamin is crucial for brain and nervous system function, including neurotransmitter synthesis.

A tingling or 'pins and needles' sensation (neuropathy) is a common neurological symptom caused by nerve damage. This happens because B12 is vital for maintaining the protective myelin sheath around your nerves.

Diagnosis typically involves a blood test to measure serum vitamin B12 levels. In some cases, a doctor may also check for levels of methylmalonic acid (MMA) and homocysteine for a more accurate assessment.

No. While dietary insufficiency is a cause for vegans and vegetarians, malabsorption is the most common reason for a deficiency. This can be caused by conditions like pernicious anemia, gastritis, or certain medications.

Treatment depends on the cause. It can involve oral supplements, B12 injections, or nasal sprays. For severe or absorption-related deficiencies, injections are often required initially, followed by long-term maintenance treatment.

If left untreated, B12 deficiency can lead to serious and potentially irreversible issues, including permanent nerve damage, persistent cognitive impairment, heart problems, and an increased risk of certain cancers.