B12 deficiency symptoms can be subtle at first and easy to attribute to other causes — making this one of the more frequently missed nutritional deficiencies in clinical practice. Vitamin B12 (cobalamin) is a water-soluble vitamin essential for red blood cell formation, neurological function, and DNA synthesis.
Because the body stores several years' worth of B12 in the liver, deficiency develops gradually, and neurological damage may become irreversible if left untreated for too long 1.
An estimated 6% of adults under 60 and nearly 20% of adults over 60 in the United States have a B12 deficiency, with higher rates in people following vegan or vegetarian diets, in those with gastrointestinal disorders, and in long-term users of certain medications 2. For a broader look at nutrients women commonly lack, see our guide to the best vitamins for women.
This page covers the full range of symptoms, the most common causes, how to interpret blood test results, and treatment options — written for adults who want to understand this condition before discussing testing with their healthcare provider.
Symptoms of Vitamin B12 Deficiency
Neurological Symptoms
Neurological damage from B12 deficiency — called subacute combined degeneration of the spinal cord in severe cases — can be among the most serious consequences and may become irreversible without prompt treatment 1. For a detailed breakdown of B12 deficiency neurological warning signs, including which symptoms signal urgent evaluation, see our dedicated guide.
Neurological symptoms include: tingling or numbness (paresthesia) in the hands and feet; balance and coordination difficulties; weakness in the legs and arms; memory problems and difficulty concentrating; depression, irritability, or personality changes; and in advanced cases, dementia-like cognitive decline and difficulty walking.
Hematological Symptoms
B12 is required for the production of red blood cells. Deficiency causes megaloblastic anemia — a condition in which red blood cells are abnormally large but functionally insufficient, unable to carry oxygen effectively 2.
Symptoms of anemia include: fatigue and weakness; pale or slightly yellowish (jaundiced) skin; shortness of breath, especially with exertion; heart palpitations and their causes; dizziness or lightheadedness; and headaches.
Oral and Gastrointestinal Symptoms
Glossitis (inflammation of the tongue) is a classic early sign of B12 deficiency — the tongue appears red, smooth, and sore, losing its normal bumpy texture. Mouth ulcers may also develop. Some individuals experience nausea, loss of appetite, constipation, or diarrhea 1.
Other Symptoms
Additional signs that may suggest B12 deficiency include: vision disturbances (optic neuropathy in severe cases); infertility (B12 is required for DNA synthesis relevant to sperm production and egg quality); elevated homocysteine (a cardiovascular risk marker that rises when B12 is low, as B12 is needed to metabolize this amino acid); and slow wound healing 2.
Fatigue and pallor from B12 deficiency can overlap with iron deficiency symptoms, and both conditions sometimes occur together — particularly in people with malabsorption disorders.
Common Causes of B12 Deficiency
B12 deficiency arises from inadequate intake, malabsorption, or increased demand.
The most common causes are: dietary insufficiency — B12 is found almost exclusively in animal products (meat, fish, eggs, dairy); strict vegans and some vegetarians are at substantial risk without supplementation 1; pernicious anemia — an autoimmune condition in which the body destroys intrinsic factor, a protein secreted by the stomach lining required for B12 absorption; atrophic gastritis — thinning of the stomach lining, common in older adults, which reduces intrinsic factor and gastric acid production; gastrointestinal surgery — gastric bypass or partial gastrectomy disrupts the absorption mechanism; Crohn's disease or celiac disease affecting the terminal ileum (where B12 is absorbed); long-term use of metformin (used for type 2 diabetes and PCOS management) — consistently associated with reduced B12 absorption 2; and long-term proton pump inhibitor (PPI) use — reduces gastric acid, impairing B12 release from food proteins.
Older adults are particularly susceptible due to multiple overlapping factors.
Diagnosis and Blood Test Interpretation
The primary diagnostic test is serum vitamin B12. A level above 300 pg/mL is generally considered normal; 200–300 pg/mL is borderline, and below 200 pg/mL is deficient 1. However, serum B12 has limitations — about 20–30% of people with normal serum B12 may still have functional deficiency.
For this reason, methylmalonic acid (MMA) and homocysteine are more sensitive functional markers — both rise when cellular B12 activity is insufficient, even when serum B12 appears adequate 2. A complete blood count (CBC) is used to assess for megaloblastic anemia (macrocytosis — enlarged red blood cells). Intrinsic factor antibody testing can diagnose pernicious anemia.
Discuss the interpretation of any results with your healthcare provider, as clinical context matters significantly.
Treatment
Treatment depends on the cause and severity of deficiency. For dietary deficiency, oral B12 supplements or B12-fortified foods are highly effective. High-dose oral B12 (1,000–2,000 mcg/day) can successfully correct deficiency even in pernicious anemia through passive diffusion absorption that bypasses intrinsic factor — though this was once thought impossible 1.
Intramuscular (IM) B12 injections (typically hydroxocobalamin or cyanocobalamin) are used when absorption is severely impaired or in cases requiring rapid repletion, such as severe neurological symptoms. IM injections are typically given daily or every other day for 1–2 weeks initially, then monthly for maintenance. Cyanocobalamin and methylcobalamin are both effective oral forms; the evidence does not strongly favor one form over the other for most people 2.
Neurological damage may begin to improve within weeks of treatment, but recovery depends on how long the deficiency was present and how severe the damage.
When to See a Doctor
Seek medical evaluation if you experience: tingling or numbness in your hands or feet that is new or worsening; balance problems or difficulty walking; memory changes, confusion, or mood disturbances; unexplained fatigue with pale skin; or a sore, smooth, red tongue. These symptoms may indicate B12 deficiency but also other serious conditions that require diagnosis.
Request B12 testing if you follow a vegan or vegetarian diet without consistent B12 supplementation; take metformin, PPIs, or H2 blockers long-term; are over 65 (routine screening is reasonable); have had gastric surgery or have a gastrointestinal disorder; or have a family history of pernicious anemia.
Our vitamin deficiency quiz can help you identify whether B12 — and other common deficiencies — may be contributing to your symptoms before seeking formal testing. Early detection and treatment can prevent permanent neurological damage.
Frequently asked questions
What is the fastest way to fix B12 deficiency?
Intramuscular B12 injections produce the fastest clinical response and are preferred for severe deficiency with neurological symptoms. For less severe deficiency without absorption problems, high-dose oral B12 (1,000–2,000 mcg/day) is highly effective and more convenient — research shows it achieves similar results to injections over several weeks to months. The best approach depends on the underlying cause of deficiency.
Can B12 deficiency cause anxiety?
B12 deficiency can affect mood and mental health through several mechanisms, including its role in synthesizing neurotransmitters such as serotonin and dopamine, and through elevated homocysteine, which is linked to depression and cognitive decline. Anxiety, irritability, and low mood can all be associated with B12 deficiency, and these symptoms may improve with treatment — though B12 is not a standalone treatment for diagnosed anxiety or depression.
How long does it take to recover from B12 deficiency?
Energy and blood markers typically begin to improve within 4 to 8 weeks of treatment. Neurological symptoms may take longer — months to a year or more — and complete recovery depends on how early treatment was started. Damage from severe, long-standing neurological involvement may not fully resolve. This is why early recognition and treatment are critical.