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Your red blood cells, nerves, and brain all depend on vitamin B12. But some people don’t get enough of this nutrient. When left untreated, a B12 deficiency can cause a number of health problems. Some are mild while others are more serious.   

Certain groups are more likely than others to have really low levels. Sometimes you may have a deficiency and not know it. You might want to ask your doctor to check your B12 levels if you fit any of the following:

  • You’re vegan or strict vegetarian.
  • You have an inflammatory bowel disease, like Crohn’s or ulcerative colitis.
  • You have celiac disease.
  • You’re older than 50.
  • You’ve had surgery on your stomach.
  • You have a family history of pernicious anemia.

A simple blood test can show if your B12 levels aren’t where they should be. Treatment, including shots or supplements, can help you get healthy. But if you ignore your deficiency, here are some of the things that can happen.


This is when your body can’t make enough healthy red blood cells. A vitamin B12 deficiency can cause megaloblastic anemia. Megaloblasts are immature red blood cells that are larger than normal. They usually have an odd shape, too.

When you have anemia, your red blood cells have trouble getting oxygen to your tissues and organs. It may take you a few years to notice any problems. But over time, you may get symptoms such as:

  • Tiredness that comes on easily
  • Weakness
  • Pale skin
  • Achiness
  • Fast or irregular heartbeat
  • Dizziness
  • Sore or red tongue
  • Enlarged liver
  • Yellow skin

If your anemia goes untreated for a long time, it can lead to heart failure. That’s because your heart has to work harder. B12-related anemia may also raise the chances you’ll have a heart attack, stroke, or other cardiovascular problem.

Nerve Problems

Anemia from a B12 deficiency can also cause issues with your nerves. This can happen with or without anemia.

B12 helps your body make myelin. That’s a protective layer around the “wires” in your nervous system. It breaks down when you don’t have enough B12, opening your nerve fibers up to damage.

At first, you may start to get a “pins and needles” feeling in your hands and feet. Your muscles may be weak. Over time, you may have numbness. This loss of sensation tends to affect your legs first.

Other symptoms include:

  • Loss of balance
  • Slow reflexes
  • Trouble walking
  • Trouble with sexual arousal
  • Bowel or bladder problems
  • Trouble with your sense of smell
  • Vision loss that gets worse over time

Many of your symptoms can get better with treatment. But some may not. It depends on how serious the nerve damage is. If left untreated for a long time, B12 deficiency can lead to paralysis that doesn’t go away.

Mental Health Issues

Mood and memory troubles are common signs of a B12 deficiency. Like nerve problems, these may show up with or without anemia.

You may get symptoms such as:

  • Depression
  • Irritability
  • Anxiety
  • Restlessness
  • Insomnia
  • Trouble thinking clearly

More serious symptoms include:

  • Trouble telling what’s real
  • Fear someone wants to hurt you
  • Seeing or hearing things that aren’t there
  • Dementia-like symptoms

Rarely, some people get extrapyramidal symptoms. That’s when your body moves in ways you can’t control. For example, you might shake or get really stiff. If these symptoms are caused by a B12 deficiency, they’ll likely get better once your levels are back to normal.

Brain Changes

While everyone’s brain shrinks with age, there’s evidence that a B12 deficiency may speed up this process. Some experts think it’s because your homocysteine levels go up when your B12 levels are too low. High levels of this amino acid are linked to brain issues and conditions like dementia and Alzheimer’s disease. Experts are still trying to figure out if B12 deficiency alone causes problems with thinking and memory.

Other Complications

Stomach cancer. If you have pernicious anemia, you don’t make enough intrinsic factor. That’s a substance that helps your stomach absorb B12 from food. Studies show people with pernicious anemia have a higher chance for developing stomach cancer.

Infertility. There’s some evidence that a lack of B12 can make it harder to get or stay pregnant. It may also raise the chances your baby will have a serious birth defect. A deficiency may also negatively affect sperm. More research is needed.

Osteoporosis. A B12 deficiency can weaken your bones. If you also have problems walking, that raises the chances you’ll break a bone if you fall.

COPD symptoms. Studies show that high homocysteine levels, which go up with a B12 deficiency, are linked with worsening chronic obstructive pulmonary disease (COPD).

Don’t Wait to Get Treatment

No matter what’s causing your deficiency, your doctor will work with you to get your levels back up. It’s important to get help early. If left untreated for too long, some of your health problems may not go away.

Show Sources


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National Organization for Rare Disorders (NORD): “Anemia, Megaloblastic.”

National Heart, Lung, and Blood Institute: “Pernicious Anemia.”

Merck Manual: “Vitamin B12 Deficiency.”

Nutrients: “Cobalamin Deficiency: Clinical Picture and Radiological Findings,” “Neuroenhancement with Vitamin B12 — Underestimated Neurological Significance.”

Clinical Medicine (Royal College of Physicians): Vitamin B12 deficiency — A 21st century perspective.”

American Family Physician: “Vitamin B12 Deficiency: Recognition and Management.”

American Journal of Preventive Medicine: “Is vitamin B12 deficiency a risk factor for cardiovascular disease in vegetarians?”

Child and Adolescent Psychiatry and Mental Health: “Mood disorder with mixed, psychotic features due to vitamin b12 deficiency in an adolescent: case report.”

Journal of Tropical Pediatrics: “Psychotic Disorder and Extrapyramidal Symptoms Associated with Vitamin B12 and Folate Deficiency.”

Neurology: “Vitamin B12, cognition, and brain MRI measures.”

Neurobiology of Aging: “Critical levels of brain atrophy associated with homocysteine and cognitive decline.”

American Cancer Risk Factors: “Stomach Cancer Risk Factors.”

Journal of Reproductive Medicine: “Vitamin B23 deficiency, infertility and recurrent fetal loss.”

NHS (UK): “Complications: Vitamin B12 or folate deficiency anaemia.”

BMC Pulmonary Medicine: “The diagnostic value of homocysteine for the occurrence and acute progression of chronic obstructive pulmonary disease.”