Written by Anne Jacobson, MD, MPH | Reviewed by Karla Robinson, MD

Updated on September 22, 2023

Key takeaways:

Vitamin B12 is needed for healthy blood cells, nerve cells, and DNA. Symptoms of vitamin B12 deficiency include anemia and neurological issues.
You can be deficient in vitamin B12 if you don’t get enough B12 in your diet or your gut can’t absorb it.
You can treat or manage vitamin B12 deficiency with supplements, injections, or nasal spray. Your provider will determine the best treatment for your vitamin B12 deficiency, based on the cause.

Table of contents
Definition
Symptoms
Causes
Diagnosis
Treatment
Bottom line
References

fizkes/iStock via Getty Images
Vitamin B12 is an essential nutrient that does its job quietly and efficiently inside the body. Vitamin B12 helps to make new DNA and keep nerve and blood cells healthy. It also helps our cells break down proteins and fats for energy.

Most people get enough vitamin B12 in their diet and absorb it through the digestive system. But some diets and certain health conditions may lead to a deficiency of this important vitamin, especially as you get older.

What is vitamin B12?
Vitamin B12, also called cobalamin, is one of the B-complex vitamins. It’s not made by the body, so we need it in our diet. Vitamin B12 is not made by plants, either. It’s found in animal products, in foods where B12 has been added, or in supplement form.

Teens and adults need about 2.4 mcg of vitamin B12 every day. Pregnant people need around 2.6 mcg, and those who are breastfeeding need 2.8 mcg of vitamin B12 per day. Infants and children need 0.4 mcg to 1.8 mcg, depending on their age.

Common symptoms of vitamin B12 deficiency
Not having enough vitamin B12 can cause complications in multiple body systems. That’s because vitamin B12 is needed for making certain cells, proper cell function, and repair. Having a vitamin B12 deficiency can affect how well your body makes new blood cells, nerve cells, and DNA.

Let’s take a closer look at four common symptoms of vitamin B12 deficiency.

Anemia
Low B12 levels cause a kind of anemia, or low blood count. Sometimes, the blood cells become large, or macrocytic, before a person becomes anemic.

Symptoms of anemia might include:

Feeling tired or having low energy
Pale skin
Feeling short of breath, especially with movement
Having less of an appetite or losing weight without trying
Headaches
Feeling dizzy or like you might faint
Fast or irregular heartbeat
Some people with B12 deficiency also have mouth sores or a swollen, painful tongue.

Nerve damage
Vitamin B12 is also important for nerve cells to function. B12 deficiency can affect the central nervous system (the brain and spinal cord) or the peripheral nerves that go to the rest of the body.

Symptoms of nerve damage from B12 deficiency can include:

Tingling, or a pins-and-needles sensation
Depression, irritability, or confusion
Changes in vision
Memory loss
Problems with balance

Birth defects
Pregnant people need to have enough B12 as well as folate, or vitamin B9, which is also an essential vitamin. If not, the baby could be born with a neural tube defect. This happens when the brain or spinal cord develops incorrectly. Vitamin B12 is also present in breast milk. So, a breastfed baby whose mother has a low B12 level could have complications. These might include delayed development, slow weight gain, or anemia.

DNA damage
Cells need vitamin B12 to make new DNA. When cells can’t make or repair DNA, they start to divide incorrectly. Low B12 and folate levels play a role in DNA damage, which may lead to diseases like cancer. This is also why low vitamin B12 is thought to play a role in heart disease, stroke, and dementia.

When should I contact my healthcare provider about vitamin B12 deficiency symptoms?
If you notice any physical symptoms of vitamin B12 deficiency, you’ll need to let your healthcare provider know right away. They will need to run some tests to see if vitamin B12 levels are the cause of your symptoms. It’s important to make the diagnosis quickly. Left untreated, low vitamin B12 levels can lead to serious neurological or physical effects.

What causes vitamin B12 deficiency?
The body stores extra vitamin B12 in the liver. So, it may take years for someone to develop symptoms from a B12 deficiency. But there are certain conditions and diets that put some people more at risk.

Not consuming enough
Most people in the U.S. consume enough vitamin B12. But you could be at risk for B12 deficiency if you follow a strict vegan or vegetarian diet without taking supplements.

Vitamin B12 comes from animal products like:

Eggs
Dairy products
Poultry
Fish
Meat
It’s added to some foods like cereals and nutritional yeast. Vitamin B12 is also present in breast milk and added to infant formula. The infants of pregnant or breastfeeding people who don’t get enough vitamin B12 are also at risk.

The amount of vitamin B12 can be found on the nutrition label of foods where it has been added and doesn’t occur naturally.

Not absorbing enough
Some people consume enough vitamin B12, but their body can’t absorb it. Some health conditions, procedures, and behaviors that may affect B12 absorption include:

Weight-loss surgery or other surgery on the stomach and intestines
Low stomach acid, occurring with age or from taking medications to reduce stomach acid.
Inflammation in the stomach or small intestine from gastritis, celiac disease, or Crohn’s disease
Pernicious anemia, an autoimmune condition where the stomach doesn’t make a protein that is needed to absorb B12
Taking metformin for diabetes, which may decrease B12 absorption
Drinking alcohol, which can interfere with the body’s ability to absorb enough B12
How is B12 deficiency diagnosed?
A healthcare provider may do blood tests if you are at risk for a vitamin B12 deficiency. A physical exam can provide clues, but you may not have any symptoms or obvious changes on an exam.

A blood test will look for anemia and the size of your blood cells. Your provider will also check your vitamin B12 levels. More specialized tests may be done for advanced disease or if autoimmune disease is suspected.

How do you treat B12 deficiency?
Depending on the severity and cause, there are oral, nasal, or injected forms of B12 treatment. If the B12 deficiency isn’t severe and is due to low intake, you can often treat it with an oral supplement.

Severe B12 deficiency — especially if it’s caused by an absorption problem or autoimmune condition — is usually treated with B12 injections. This may be for a period of time, or it may be for life, depending on the condition. There is also a nasal spray, but it’s not absorbed as well as the injectable form.

If medications or alcohol are interfering with absorption, these should be limited or stopped whenever possible.

How long does it take to recover from B12 deficiency?
After treatment, your blood count may start to improve after several days. Nerve damage may take longer to heal and could be permanent if it’s not treated within the first 6 months of symptoms.

How can I prevent vitamin B12 deficiency?
Not all forms of B12 deficiency can be prevented. But there are some things you can do to decrease your risk. These include:

Eating a balanced diet: This means including foods that are fortified with B12 or supplements
Drinking in moderation: This means 1 drink a day for women and 2 a day for men.
Getting screened: This is important if you take medications or have a condition that puts you at risk
Can you take too much B12?
Vitamin B12 supplements are sometimes marketed as a way to boost energy. But if you have normal levels in your body already, taking more won’t improve your energy level. Although B12 is generally considered safe, taking large amounts of supplements may have unwanted side effects.

Side effects can include:

Nausea
Diarrhea
Rash
Cough and congestion
The bottom line
Vitamin B12 deficiency can happen in people who don’t get enough B12 in their diet or can’t absorb it well. Serious side effects can result. In severe cases, the damage is permanent. If you know the symptoms and risk factors for B12 deficiency, though, it’s a condition that can be easily treated and managed.

References
Allen, L. H. (2012). Vitamin B-12. Advances in Nutrition.

Aroda, V. R., et al. (2016). Long-term metformin use and vitamin B12 deficiency in the Diabetes Prevention Program Outcomes Study. The Journal of Clinical Endocrinology and Metabolism.