Vitamin B12 deficiency causes problems across nearly every system in your body, from your blood cells and nerves to your mood and skin. Because your body stores B12 in the liver, deficiency develops slowly, sometimes taking years before symptoms appear. That slow progression is part of what makes it dangerous: by the time you notice something is wrong, the damage can be significant. Blood levels below 200 pg/mL are generally considered deficient, though neurological and psychiatric symptoms can appear even when levels fall in the low-normal range, between 200 and 300 pg/mL.
Nerve Damage and Numbness
B12 plays a direct role in maintaining myelin, the protective coating around your nerves. Without enough B12, myelin layers begin to swell and split apart, creating patches of damage along the spinal cord’s white matter. This process doesn’t happen uniformly. Some nerve fibers deteriorate while neighboring ones remain intact, which is why early neurological symptoms can feel inconsistent or come and go.
The damage typically starts with tingling or numbness in your hands and feet. As it progresses, you may notice difficulty with balance, a feeling of weakness in your legs, or trouble coordinating movements. In more advanced cases, people lose the ability to sense vibration or know where their limbs are in space without looking. These neurological symptoms are classified as “subacute combined degeneration” and tend to develop later and more gradually than the blood-related effects of deficiency. Up to 20% of people with neurological damage from B12 deficiency show no anemia at all, which means relying on blood counts alone can miss the problem entirely. Delayed diagnosis can lead to progressive and irreversible nerve damage.
Anemia and Blood Cell Changes
B12 is essential for making healthy red blood cells. When levels drop, your bone marrow produces red blood cells that are abnormally large and oval-shaped, a condition called megaloblastic anemia. Normal red blood cells have a volume between 80 and 100 femtoliters. In B12 deficiency, that volume can balloon past 115 fl, sometimes reaching 134 fl or higher. These oversized cells don’t function well and break down faster than normal ones.
Macrocytosis, the enlargement of red blood cells, is typically the earliest detectable change on a routine blood test. Along with the large oval cells, white blood cells called neutrophils develop extra segments in their nuclei, a hallmark finding that helps distinguish B12 deficiency from other causes of anemia. The resulting symptoms are what you’d expect from any anemia: fatigue, shortness of breath, pale skin, and lightheadedness.
Psychiatric and Cognitive Effects
B12 deficiency can affect your brain in ways that look nothing like a vitamin problem. Psychiatric symptoms include depression, apathy, irritability, and in severe cases, hallucinations, catatonia, or delirium. These aren’t rare outliers. Up to 28% of people with B12 deficiency develop neuropsychiatric symptoms before any changes show up in their blood counts or red blood cell size, the markers most commonly checked during routine screening.
Cognitive decline is another concern, particularly in older adults. Memory problems, difficulty concentrating, and confusion can mimic early dementia. In some cases, treating the deficiency reverses these symptoms, though the longer they persist before treatment, the less likely full recovery becomes.
Cardiovascular Risk
B12 helps your body break down an amino acid called homocysteine. When B12 is low, homocysteine accumulates in your blood, and elevated homocysteine has a well-documented link to heart disease. One key mechanism: homocysteine reduces your liver’s production of HDL cholesterol (the protective kind) and speeds up its clearance from your bloodstream. Low HDL is one of the strongest predictors of premature cardiovascular disease across population studies. Homocysteine also appears to promote the buildup of fatty deposits in artery walls, a process that underlies heart attacks and strokes.
Mouth and Skin Changes
Your tongue and skin can show visible signs of B12 deficiency. About half of people with pernicious anemia, one of the most common causes of severe deficiency, develop a smooth, beefy-red tongue with flattened or absent papillae. The tongue may also be painful.
Skin hyperpigmentation follows a distinctive pattern. It tends to be most pronounced on the hands and feet, particularly over the backs of the fingers and toes, with darker coloring concentrated at the joints. On the palms, the darkening is usually diffuse, while on the soles it appears mottled. This pattern is distinctive enough that dermatologists sometimes use it as a diagnostic clue.
Risks During Pregnancy
Low B12 in women around the time of conception significantly raises the risk of neural tube defects in the baby. These birth defects affect the brain and spinal cord and include spina bifida, which can cause partial paralysis, and anencephaly, a fatal condition where the brain and skull fail to develop properly. Research from the National Institute of Child Health and Human Development found that women with B12 levels in the deficient range (below 150 ng/L) had five times the risk of having a child with a neural tube defect compared to women with the highest levels. Even moderately low levels carried 2.5 to 3 times the risk. The researchers recommended that women have B12 levels above 300 ng/L before becoming pregnant.
Why Deficiency Develops
B12 absorption requires a protein called intrinsic factor, produced by cells in your stomach lining. In pernicious anemia, your immune system attacks those cells or the intrinsic factor itself, blocking absorption regardless of how much B12 you eat. This autoimmune condition is one of the most common causes of severe deficiency.
Other causes include a strictly plant-based diet (B12 is found almost exclusively in animal products), long-term use of acid-reducing medications, and gastrointestinal conditions that impair absorption, such as celiac disease or Crohn’s disease. Older adults are at higher risk because stomach acid production declines with age, making it harder to release B12 from food. Because the liver stores substantial amounts of B12, it can take up to 10 years of inadequate intake before overt deficiency symptoms emerge.
How Deficiency Is Confirmed
A standard serum B12 test is the usual starting point, with levels below 200 to 250 pg/mL considered subnormal. But this test alone can miss people who are functionally deficient. When B12 levels fall between 150 and 399 pg/mL, a more sensitive test measures methylmalonic acid (MMA), a substance that accumulates when B12 is insufficient. MMA levels above 0.271 micromol/L suggest deficiency, and the test has a reported sensitivity of 99.8%, making it the gold standard for confirming the diagnosis when serum B12 results are ambiguous.