What Happens When You Don't Have Vitamin B12 in Your Body?

December 11, 2025 •

2 min read

According to the National Institutes of Health, the body's store of vitamin B12 can take years to be depleted, but when it does, the effects can be severe and widespread. When you don't have vitamin B12 in your body, it can lead to a cascade of health issues affecting your blood, nervous system, and cognitive function.

Quick Summary

A lack of vitamin B12 disrupts red blood cell production, causing anemia and damaging the nervous system, leading to fatigue, nerve pain, cognitive decline, and mood disturbances.

Key Points

Blood Cell Production Failure: Without B12, the body cannot produce enough healthy red blood cells, leading to megaloblastic anemia, which causes fatigue, weakness, and paleness.
Neurological Damage Risk: A lack of B12 can damage the protective myelin sheath covering nerves, causing tingling, numbness, balance problems, and cognitive decline.
Psychological Symptoms: B12 deficiency can contribute to mood changes, including depression, irritability, confusion, and, in severe cases, more serious psychiatric conditions.
Irreversible Effects: If neurological issues caused by deficiency are left untreated for too long, the damage may become permanent.
At-Risk Groups: Individuals who follow strict vegan diets, those with autoimmune conditions like pernicious anemia, and older adults are at higher risk of B12 deficiency.
Treatment Options: Treatment typically involves supplementation, either through oral tablets or injections, depending on the cause and severity of the deficiency.

The Core Functions of Vitamin B12

Vitamin B12, also known as cobalamin, is an essential water-soluble nutrient vital for numerous bodily processes. It is crucial for the formation of red blood cells, DNA synthesis, and maintaining the health of the nervous system, including the proper functioning of nerve cells. When dietary intake or absorption is insufficient, the body’s reserves are eventually exhausted, leading to a host of symptomatic issues that can be debilitating if left unaddressed.

Blood-Related Consequences: Anemia and Beyond

Severe vitamin B12 deficiency can lead to megaloblastic anemia, where the body produces abnormally large, dysfunctional red blood cells unable to carry oxygen efficiently. This results in chronic fatigue, weakness, pale or jaundiced skin, and shortness of breath and dizziness.

Neurological Damage and Its Wide-Ranging Effects

A serious long-term consequence of untreated vitamin B12 deficiency is nervous system damage due to its role in myelin production. Symptoms include peripheral neuropathy (tingling/numbness in extremities), balance and coordination problems (ataxia), and cognitive issues such as memory loss, confusion, and difficulty concentrating. Psychological effects can involve irritability, depression, anxiety, or even paranoia and dementia. Vision problems are a rare but possible symptom.

Gastrointestinal and Other Symptoms

Other symptoms of B12 deficiency can include a sore, red, and inflamed tongue (glossitis), and digestive issues like changes in appetite, nausea, weight loss, or altered bowel habits.

Comparing Causes of Vitamin B12 Deficiency


Cause	Mechanism	At-Risk Population	Treatment Approach
Dietary Insufficiency	Inadequate intake of B12 from food sources.	Vegans, vegetarians, older adults with poor diets.	Oral supplements, fortified foods, dietary changes.
Pernicious Anemia	Autoimmune condition preventing the production of Intrinsic Factor, a protein required for B12 absorption.	People of Northern European descent, those with other autoimmune diseases.	Lifelong B12 injections or high-dose oral supplements.
Malabsorption Issues	Gastrointestinal conditions like Crohn's disease, celiac disease, or prior stomach/intestinal surgery impair absorption.	People with specific digestive disorders or history of certain surgeries.	Regular B12 injections to bypass the absorption issue.
Certain Medications	Some drugs, such as metformin and acid-reducing medications, interfere with B12 absorption.	Individuals on long-term medication use, especially for diabetes or acid reflux.	Supplementation, often in high doses, while continuing medication.

Conclusion

Vitamin B12 is essential for healthy blood cell production and nervous system function. Lacking it can cause effects from mild fatigue and mood changes to potentially irreversible nerve damage. While the body stores B12, long-term deficiency is serious. Early detection and treatment are crucial, particularly for vegans, older adults, and those with digestive issues. Screening and supplementation can prevent complications. For more on neurological effects, consult resources like the National Center for Biotechnology Information.

National Center for Biotechnology Information

Frequently Asked Questions

Symptoms of a B12 deficiency can take a long time to emerge, sometimes several years, because the body stores a substantial amount of the vitamin in the liver.

Early signs can be subtle and non-specific, including mild fatigue, weakness, headaches, and a sore or red tongue. Some individuals may not experience noticeable symptoms initially.

Yes, a B12 deficiency can cause a variety of psychological issues, including depression, irritability, mood swings, and, in advanced cases, more serious cognitive problems and memory loss.

Diagnosis typically involves a blood test to measure your B12 levels. Further tests, such as checking for elevated methylmalonic acid (MMA) or homocysteine, may be used if the B12 level is borderline.

At-risk groups include vegans and strict vegetarians, older adults, individuals with autoimmune diseases like pernicious anemia, and people with gastrointestinal disorders like Crohn's or celiac disease.

Treatment depends on the cause. For dietary deficiencies, oral supplements may be sufficient, while issues with absorption often require regular B12 injections. High-dose oral therapy can also be effective for some.

While many symptoms, including nerve-related ones, can improve with treatment, neurological damage that has lasted for an extended period may be irreversible.