What are the side effects of having low B12?

October 19, 2025 •

4 min read

According to the National Institutes of Health, vitamin B12 deficiency can go unnoticed for years due to the body's large storage capacity in the liver, as reported by MSD Manuals. However, once stores are depleted, the side effects of having low B12 can manifest as a wide range of physical, neurological, and psychological symptoms that gradually worsen over time if untreated.

Quick Summary

This article explores the comprehensive side effects of low B12, detailing the physical, neurological, and psychological symptoms associated with the deficiency, and explaining potential complications if left untreated.

Key Points

Neurological damage: Low B12 can cause numbness, tingling, and difficulty with balance and coordination, which may become permanent if untreated.
Cognitive impairment: Forgetfulness, confusion, and difficulty thinking are common side effects, potentially progressing to dementia.
Psychological problems: Mood changes such as depression, irritability, and in severe cases, psychosis and paranoia, can result from low B12.
Megaloblastic anemia: Deficiency can lead to a lack of healthy red blood cells, causing fatigue, weakness, and a pale complexion.
Diagnosis is key: Because symptoms are non-specific and develop slowly, laboratory tests are essential for confirming a B12 deficiency.
Treatment can reverse effects: Many side effects are reversible with B12 supplementation, especially when caught early, though long-standing nerve damage may persist.

Understanding Vitamin B12 Deficiency

Vitamin B12, or cobalamin, is a vital nutrient for several key bodily functions, including the production of DNA and the formation of red blood cells. A deficiency occurs when the body does not get enough of the vitamin from diet or cannot absorb it properly from the digestive system. Conditions like pernicious anemia, gastritis, or certain surgeries can interfere with absorption. The gradual onset of symptoms means that the side effects of low B12 can be subtle at first, becoming more severe as the deficiency progresses. Recognizing these symptoms is crucial for early intervention and preventing potentially permanent health issues.

Common Physical Symptoms

The physical effects of low B12 are often linked to the impact on red blood cell production, which can lead to a type of megaloblastic anemia. With inadequate B12, the body produces abnormally large, immature red blood cells that cannot function correctly, reducing oxygen delivery throughout the body. This can result in a number of common and recognizable symptoms.

General and Gastrointestinal Symptoms

Fatigue and Weakness: A lack of healthy red blood cells can cause extreme tiredness and lack of energy, as the body's tissues and organs don't receive enough oxygen.
Pale or Yellowish Skin: Anemia and the destruction of large, fragile red blood cells can lead to a pale complexion or a slight yellowish tinge to the skin and eyes.
Glossitis and Mouth Ulcers: A smooth, red, and swollen tongue (glossitis) is a classic sign of B12 deficiency, along with the appearance of mouth ulcers.
Digestive Issues: Many individuals experience gastrointestinal problems such as nausea, vomiting, diarrhea, or a loss of appetite and unexplained weight loss.
Rapid Heartbeat and Breathlessness: In more severe cases of anemia, the heart may beat faster to compensate for the reduced oxygen-carrying capacity of the blood, leading to palpitations and shortness of breath.

Neurological and Psychological Manifestations

Vitamin B12 plays a critical role in maintaining a healthy nervous system. When levels are low, it can lead to a range of neurological symptoms, some of which may become irreversible if not treated promptly. The psychological effects can also be significant, impacting mood and mental clarity.

Neurological Symptoms

Numbness and Tingling: A hallmark neurological symptom is a pins-and-needles sensation, numbness, or tingling in the hands, legs, and feet, known as paresthesia.
Balance and Coordination Issues: Damage to the nervous system, particularly the spinal cord, can affect balance and coordination, making it difficult to walk steadily. This condition is sometimes referred to as ataxia.
Vision Problems: Disturbed vision, including blurred vision or changes to eyesight, can occur due to damage to the optic nerve.

Cognitive and Psychological Changes

Memory Problems and Confusion: Many people with low B12 experience cognitive difficulties, such as memory loss, confusion, and slower thinking. In advanced cases, this can lead to dementia.
Mood Swings and Irritability: Psychological symptoms often include mood changes such as depression, irritability, and anxiety.
Psychosis and Paranoia: In severe cases, a B12 deficiency can lead to more serious psychological issues like paranoia, delusions, and hallucinations.

Low B12 vs. Folate Deficiency: A Comparison

Because B12 and folate deficiencies can cause a similar type of anemia and share some overlapping symptoms, distinguishing between them is important for proper treatment. Key differences lie in specific metabolic pathways and potential for neurological damage.


Feature	Low B12 Deficiency (Cobalamin)	Folate Deficiency (B9)
Associated Anemia	Macrocytic, megaloblastic anemia	Macrocytic, megaloblastic anemia
Neurological Symptoms	Common and potentially irreversible, including nerve damage, numbness, and cognitive issues	Uncommon; primarily affects red blood cells and not typically the nervous system directly
Elevated Metabolites	Elevated methylmalonic acid (MMA) and homocysteine	Elevated homocysteine, but normal MMA
Primary Cause	Often malabsorption (e.g., pernicious anemia) or dietary	Often dietary insufficiency, alcoholism, or poor absorption
Treatment Response	Requires B12 supplementation; neurological symptoms may not fully resolve if long-standing	Responds well to folic acid tablets

Serious Long-Term Complications

Ignoring the side effects of low B12 can lead to serious and potentially irreversible complications.

Permanent Nerve Damage: If left untreated for a prolonged period, the neurological damage (neuropathy) caused by B12 deficiency can become permanent.
Heart and Vascular Conditions: Elevated levels of homocysteine resulting from low B12 are a risk factor for heart disease and stroke. Severe anemia can also lead to heart failure as the heart works harder to pump blood.
Pregnancy Complications: In pregnant women, low B12 levels are associated with a higher risk of birth defects, such as neural tube defects, and pregnancy complications like premature birth.
Increased Cancer Risk: Individuals with pernicious anemia, a common cause of B12 deficiency, have an increased risk of developing certain gastric cancers.

When to Seek Medical Attention

If you experience persistent symptoms associated with B12 deficiency, especially neurological ones like numbness, tingling, or memory issues, it is essential to see a healthcare professional. Early diagnosis and treatment are the best ways to prevent long-term damage. Your doctor can perform blood tests to check your B12 and folate levels and determine the underlying cause. Treatment often involves supplements or injections to restore B12 levels. For more information on vitamin deficiency anemia, you can refer to the National Heart, Lung, and Blood Institute (NHLBI) at the National Institutes of Health: NHLBI, NIH.

Conclusion: The Importance of Addressing Low B12

The side effects of having low B12 are wide-ranging and can affect virtually every system in the body, from physical energy and digestive health to mental and neurological function. While the body's natural storage can mask the issue for some time, delaying treatment risks permanent and disabling complications, including nerve damage and serious cardiovascular problems. Given the potential severity, individuals experiencing persistent symptoms of fatigue, neurological issues, or mood changes should consult a doctor to get tested for B12 deficiency and begin treatment promptly if needed.

Frequently Asked Questions

The initial signs of low B12 often include general fatigue, weakness, lack of energy, and feeling tired easily. These can be subtle and develop over a long period, making them easy to dismiss.

Yes, a severe or long-term vitamin B12 deficiency can cause nerve damage, or neuropathy. This can result in symptoms such as numbness, tingling in the hands and feet, and problems with walking and balance.

Low B12 can lead to various mental health issues, including mood swings, depression, and irritability. In very advanced cases, it can cause more severe psychological problems like paranoia, delusions, and confusion.

Low B12 impairs the production of healthy red blood cells, leading to a condition called megaloblastic anemia. The red blood cells that are produced are abnormally large and don't function correctly, which reduces oxygen delivery throughout the body.

Yes, it is possible to have a vitamin B12 deficiency without developing anemia. Some people experience neurological symptoms, like nerve damage, before any signs of anemia become apparent.

While both can cause a similar type of anemia, low B12 uniquely causes neurological problems, including nerve damage, which is not typically seen with a folate deficiency. Lab tests can also distinguish between them by measuring methylmalonic acid (MMA) levels.

Many symptoms of B12 deficiency are reversible with appropriate treatment. However, if severe neurological damage has occurred due to a long-standing and untreated deficiency, some of the damage may be permanent.

Individuals at a higher risk include strict vegetarians and vegans, older adults, people with digestive diseases like Crohn's or celiac disease, and those with autoimmune conditions such as pernicious anemia.