What happens to your body when your B12 is very low?

October 11, 2025 •

2 min read

According to the NIH Office of Dietary Supplements, the symptoms of vitamin B12 deficiency can take several years to appear because the body stores a substantial amount. Understanding what happens to your body when your B12 is very low is crucial for preventing severe, long-term health complications.

Quick Summary

A severe B12 deficiency can cause significant issues, from chronic fatigue and paleness to peripheral neuropathy and impaired cognitive function. It can lead to megaloblastic anemia and permanent nerve damage if untreated. Diagnosis involves blood tests, and treatment typically includes supplements or injections.

Key Points

Nervous System Damage: A severe lack of B12 can damage the myelin sheath on nerve fibers, leading to symptoms like tingling, numbness, and balance problems.
Megaloblastic Anemia: Very low B12 impairs red blood cell production, causing abnormally large red blood cells that lead to chronic fatigue, weakness, and paleness.
Cognitive Decline: Severe deficiency can manifest as psychological issues including confusion, memory loss, depression, and, in advanced cases, dementia.
Gastrointestinal Distress: Physical symptoms can include a sore tongue, loss of appetite, weight loss, and digestive problems like diarrhea.
Irreversible Consequences: If left untreated, severe B12 deficiency can cause permanent neurological damage and other complications that do not improve even with supplementation.

Neurological Impacts of Very Low B12

A critical function of vitamin B12 is its role in forming myelin, the protective nerve sheath. A severe deficiency causes myelin degeneration, leading to subacute combined degeneration of the spinal cord and a range of neurological issues.

Neurological Symptoms Include:

Sensory changes: Numbness, tingling (pins and needles), or loss of sensation in limbs.
Coordination problems: Difficulty with balance and walking (ataxia).
Vision issues: Blurred or disturbed vision due to optic nerve damage.
Cognitive and mood changes: Confusion, memory loss, depression, paranoia, and potential dementia.
Muscle weakness: Reduced muscle strength and reflexes.

Hematological and Physical Effects

Severe B12 deficiency impairs healthy red blood cell production, resulting in megaloblastic anemia. This condition is characterized by abnormally large, ineffective red blood cells, reducing oxygen transport.

Physical Symptoms and Complications Include:

Fatigue: Extreme tiredness and weakness due to reduced oxygen delivery.
Skin changes: Paleness from lack of red blood cells or jaundice from their breakdown.
Tongue inflammation: A sore, smooth, and painful tongue (glossitis).
Digestive problems: Diarrhea, nausea, vomiting, or appetite loss.
Heart strain: Severe anemia can stress the heart, potentially causing palpitations or rapid heartbeat.

Severe vs. Mild B12 Deficiency


Feature	Mild to Moderate B12 Deficiency	Very Low (Severe) B12 Deficiency
Symptom Onset	Gradual and often subtle.	Can be rapid and severe.
Anemia Impact	May involve mild fatigue.	Typically significant megaloblastic anemia with pronounced weakness.
Nervous System	Mild tingling; potentially reversible.	Severe nerve damage, potentially permanent (ataxia, vision loss, dementia).
Psychological	Mild anxiety or depression.	Significant confusion, paranoia, and memory issues.
Treatment Needs	Often oral supplements.	Urgent treatment, often injections, to prevent permanent harm.

The Urgency of Treatment

Prompt diagnosis and treatment are critical as severe, untreated deficiency can lead to irreversible damage, especially to the nervous system. Treatment often involves B12 injections for severe cases or malabsorption, bypassing digestive issues for faster results. Conditions like pernicious anemia require lifelong B12 therapy.

Conclusion

A very low vitamin B12 level is a serious health concern impacting multiple bodily systems. It can cause debilitating fatigue, significant neurological damage, and severe psychological changes. While the body's B12 stores can initially mask the problem, symptomatic deficiency often indicates a severe issue requiring immediate medical attention. Early intervention is key to preventing long-term complications and permanent damage. For additional resources on dietary B12, consult the NIH Office of Dietary Supplements: https://ods.od.nih.gov/factsheets/VitaminB12-Consumer/.

Frequently Asked Questions

The most immediate threat is severe neurological damage. Vitamin B12 is vital for nerve function, and a severe deficiency can lead to permanent nerve damage, affecting sensation, balance, and cognitive function.

Yes, a severe B12 deficiency can cause significant cognitive issues, including confusion, memory problems, and even dementia-like symptoms, particularly in older adults.

A severe deficiency is typically diagnosed with blood tests that measure the level of vitamin B12 in the blood. Elevated levels of methylmalonic acid (MMA) can also indicate a B12 deficiency.

The fastest method is through intramuscular B12 injections. This method is preferred for severe cases and those with malabsorption issues, as it bypasses the digestive system and rapidly increases vitamin levels.

Yes. It is possible to have a B12 deficiency that causes significant nerve damage (neuropathy) even before megaloblastic anemia develops.

Risk factors include older age, following a strict vegan diet without supplementation, having a digestive disease like Crohn's or Celiac, prior gastrointestinal surgery, or an autoimmune condition like pernicious anemia.

Some symptoms, like those related to anemia, are often reversible with treatment. However, severe or long-lasting nerve damage can be permanent, especially if treatment is delayed.