Is feeling sick a symptom of B12 deficiency?

January 7, 2026 •

3 min read

According to Johns Hopkins Medicine, common symptoms of vitamin B12 deficiency can include physical effects like nausea and decreased appetite. While many associate this condition with fatigue, the impact on the digestive system can also cause you to feel sick.

Quick Summary

Nausea, reduced appetite, and other gastrointestinal issues can indicate a B12 deficiency. Symptoms often progress slowly, ranging from tiredness to more serious neurological and psychological effects if left untreated. Proper diagnosis via blood tests is crucial.

Key Points

Feeling Sick is a Symptom: Nausea, vomiting, and reduced appetite can indicate a vitamin B12 deficiency, often due to its effect on the digestive tract and red blood cell production.
Beyond Digestive Issues: Common symptoms also include fatigue, weakness, pale skin, shortness of breath, and headaches.
Neurological Complications: A key differentiator from other deficiencies is the potential for nerve damage, causing pins and needles, balance issues, and memory problems.
Causes Vary: Reasons for deficiency range from dietary insufficiency (especially in vegans) to absorption problems caused by conditions like pernicious anemia or specific medications.
Early Diagnosis is Critical: Because some severe neurological effects can be permanent, it's important to see a doctor for blood tests to diagnose and start treatment early.
Treatment is Manageable: Depending on the cause, B12 deficiency can be treated with oral supplements, fortified foods, or regular injections.

B12 Deficiency's Impact on the Digestive System

Yes, feeling sick, including symptoms like nausea, vomiting, and diarrhea, can indeed be a symptom of a B12 deficiency. Vitamin B12 is essential for the healthy function of many body systems, and when levels are low, it can disrupt normal processes, particularly those involving the digestive system and red blood cell production. In cases where a deficiency leads to anemia (megaloblastic anemia), the gastrointestinal tract can be affected, leading to a range of unpleasant symptoms.

How B12 Deficiency Causes Digestive Distress

Low levels of B12 can lead to digestive issues through several mechanisms:

Anemia's Effect: B12 is needed for red blood cell production. Megaloblastic anemia due to deficiency means fewer healthy red cells, potentially irritating the digestive tract lining and causing nausea, vomiting, and diarrhea.
Intrinsic Factor: A common cause of deficiency is poor absorption, often due to a lack of intrinsic factor. Conditions like pernicious anemia or chronic gastritis affect stomach function and intrinsic factor production, leading to B12 malabsorption and digestive upset.
Glossitis: A sore, swollen tongue is a known symptom and can contribute to a general feeling of sickness.

Common Symptoms Beyond Feeling Sick

B12 deficiency often causes symptoms that develop gradually and can vary widely. Awareness of the full range is important.

Physical Symptoms

Fatigue: Extreme tiredness is a hallmark symptom, resulting from inadequate red blood cells to carry oxygen.
Skin Changes: Pale or a yellowish (jaundiced) skin tone can occur.
Headaches and Dizziness: Reduced oxygen delivery to the brain may cause these symptoms.
Cardiovascular Signs: Shortness of breath and a fast heartbeat can be signs the body is compensating for low red blood cells.

Neurological Symptoms

Tingling: Pins and needles sensations in extremities are common due to nerve damage.
Cognitive Issues: Confusion, poor memory, and reduced mental function are possible.
Motor Control: Difficulty with balance and coordination can occur from nerve effects.
Vision: Some individuals report blurred or disturbed vision.

Comparison of B12 and Folate Deficiency Symptoms

Distinguishing between B12 and folate deficiency is important, as both can cause megaloblastic anemia and some overlapping symptoms.


Symptom	B12 Deficiency	Folate Deficiency
Feeling Sick (Nausea/Vomiting)	Common, often related to digestive upset.	Less common, but general malaise can occur.
Tingling/Numbness	A defining neurological symptom (pins and needles).	Very rare; typically not a feature unless B12 is also low.
Cognitive Issues	Common, including memory loss and confusion.	Possible, but usually less severe than with B12.
Fatigue/Weakness	Common, due to anemia.	Common, due to anemia.
Anemia	Yes, megaloblastic anemia.	Yes, megaloblastic anemia.
Source	Animal products (meat, dairy) and fortified foods.	Green vegetables, fruits, fortified grains.

The Role of Diagnosis and Treatment

Due to varied and subtle symptoms, diagnosing B12 deficiency requires medical evaluation. Blood tests measuring B12, methylmalonic acid (MMA), and homocysteine levels are typically used.

Treatment varies based on the cause. Dietary deficiency may be managed with supplements or diet changes. Malabsorption issues like pernicious anemia often require ongoing B12 injections. Timely diagnosis and treatment are crucial to prevent irreversible neurological damage.

Conclusion

Feeling sick, particularly with digestive issues like nausea and loss of appetite, can be a symptom of vitamin B12 deficiency, often linked to megaloblastic anemia. However, symptoms are diverse, including fatigue, neurological problems, and mood changes, overlapping with other conditions. Therefore, professional medical diagnosis through blood tests is essential. Prompt treatment can effectively manage symptoms and prevent long-term complications.

Learn more about B12 deficiency from a trusted source like the Cleveland Clinic.

Frequently Asked Questions

Yes, B12 deficiency can cause chronic nausea, as well as vomiting and decreased appetite, due to its effect on the digestive system and red blood cells.

B12 deficiency can cause inflammation of the stomach lining (gastritis) and lead to malabsorption issues. These, in turn, can result in digestive symptoms like nausea, diarrhea, and heartburn.

The initial symptoms often include fatigue, weakness, paleness, and shortness of breath, which develop slowly and can be easy to overlook.

Pernicious anemia, a common cause of B12 deficiency, can certainly cause you to feel sick. Symptoms include nausea, bloating, and loss of appetite.

If nausea is caused by a B12 deficiency, treatment with injections or supplements can often resolve the symptoms as vitamin levels are restored. However, it is vital to have the condition diagnosed by a doctor first.

Yes, a B12 deficiency must be confirmed through blood tests. This is crucial for proper diagnosis and to rule out other conditions with similar symptoms.

Foods rich in vitamin B12 include animal products like meat, fish (salmon, tuna, clams), milk, eggs, and cheese. Fortified foods such as cereals and nutritional yeast are also good sources.