Vitamin B12 shots: Uses, benefits, and side effects


Vitamin B12 shots are injections that can be prescribed to treat a vitamin B12 deficiency.

A deficiency in this essential nutrient can lead to an array of health problems, ranging from fatigue to permanent neurological changes.

This article addresses the importance of maintaining adequate B12 levels and outlines the benefits and risks associated with B12 shots.

Contents of this article:

  1. What is vitamin B12?
  2. Who should consider B12 shots?
  3. Benefits of vitamin B12 shots
  4. Side effects and potential risks

What is vitamin B12?

Vitamin B12 can be taken in the form of injections.

Vitamin B12 is an essential water-soluble vitamin that plays an important role in many functions in the body, including:

  • DNA synthesis
  • energy production
  • nerve cell health
  • red blood cell formation
  • neurological function

B12 is present in many food sources (mainly animal-based), where it is bound to the protein molecules in that food.

Vitamin B12 is separated from the protein during digestion and is absorbed into the bloodstream.

Adequate stomach acid is required to release the vitamin from the protein, and a substance called intrinsic factor is necessary to ensure its absorption.

People who are unable to absorb vitamin B12 properly may have pernicious anemia, which is a type of anemia characterized by a lack of intrinsic factor.

The average daily intake of vitamin B12, as recommended by the Office of Dietary Supplements, is 2.4 micrograms (mcg) for men and women over 14. Pregnant and breastfeeding women require slightly more, at 2.6 mcg and 2.8 mcg, respectively.

B12 shots: Synthetic vitamin B12

In addition to getting B12 from food sources, it is possible to acquire B12 through a man-made version of the nutrient, known as cyanocobalamin.

Vitamin B12 shots are injections containing high levels of cyanocobalamin. These shots, which can be self-administered or given by a doctor, can quickly boost B12 levels in someone who is deficient.

Cyanocobalamin is available in liquid, tablet, or capsule form. Certain foods, such as cereals and non-dairy milk and yogurts, may be fortified with the man-made form of vitamin B12.

Who should consider B12 shots?

While B12 supplements and foods fortified with the vitamin are readily available, vitamin B12 shots are only available by prescription, so a clinical diagnosis is always necessary.

It is important to note that low levels are rare in most healthy adults because the human liver stores several years’ worth of vitamin B12.

However, some groups of people are more at risk of deficiency than others and may wish to discuss the possibility of B12 shots with their doctor. These at-risk groups include:

People with signs and symptoms of B12 deficiency

Anyone who is displaying any of the signs and symptoms of a B12 deficiency or pernicious anemia should consult their doctor immediately.

Some common signs and symptoms include:

  • decreased cognitive function, such as issues with memory or understanding
  • fatigue
  • lethargy
  • constipation
  • feeling faint
  • depression or irritability
  • headache
  • difficulty maintaining balance
  • sore, swollen tongue, which may be pale yellow or very red
  • heart palpitations
  • mouth ulcers
  • paraesthesia (pins and needles)
  • vision changes

B12 deficiency risk factors

Both diabetes and some medications for type 2 diabetes can increase the risk of vitamin B12 deficiency.

Also, the following risk factors can increase the chance of developing vitamin B12 deficiency:

  • alcohol abuse
  • smoking
  • certain prescription medications, including antacids and some type 2 diabetes drugs
  • having an endocrine-related autoimmune disorder, such as diabetes or a thyroid disorder
  • eating a vegetarian or vegan diet
  • certain gastrointestinal disorders, such as Crohn’s disease
  • gastric bypass or the removal of parts of the stomach
  • aging

People with gastrointestinal issues

A gastrointestinal tract that does not function normally may inhibit vitamin B12 release or absorption. For this reason, people with disorders such as celiac disease and Crohn’s disease may be at higher risk of vitamin B12 deficiency.

Individuals who have undergone gastrointestinal surgery, including weight loss surgery, may have less of the cells necessary to secrete stomach acid and intrinsic factor. As a result, they may struggle to absorb B12.

Vitamin B12 shots, rather than oral supplements, may be particularly helpful for people who have gastrointestinal issues because injections tend to bypass these areas.

Older adults

Research suggests that vitamin B12 deficiency affects a greater number of older adults than those under the age of 65.

Older adults can be affected by conditions that are linked to a decrease in stomach acid production, including inflammation in the stomach (atrophic gastritis).

Furthermore, lower levels of stomach acid can encourage the growth of certain bacteria in the gut that use B12, reducing the amount of the vitamin available to the body.

The Institute of Medicine recommend that adults over 50 meet their vitamin B12 needs with fortified foods, B12 shots, or other supplements, as these man-made forms appear to be more readily absorbed than naturally occurring B12.

Vegetarians and vegans

As vitamin B12 is found mainly in fish, meat, eggs, and dairy, people who do not consume these foods are at risk of B12 deficiency and may benefit from regular B12 shots or other supplements.

Research published in the European Journal of Clinical Nutrition indicates that vegans and some strict vegetarians have low levels of vitamin B12.

Of 232 vegans studied, over half were classed as being deficient in vitamin B12. The deficiency was observed in just 7 percent of the vegetarians studied, and in only one of the omnivores.

Vegetarian or vegan women who are pregnant will need to be especially careful about supplementing or consuming fortified foods, as vitamin B12 transfers to the baby via the placenta and breast milk.

Infants who have vitamin B12 deficiency can experience permanent and severe neurological issues.

Benefits of vitamin B12 shots

There are several reasons to consider getting a B12 shot, including:

Reduced risk of vitamin B12 deficiency and associated complications

The most obvious benefit of receiving vitamin B12 shots is treating a vitamin B12 deficiency and avoiding its associated symptoms.

In addition, B12 shots reduce the risk of some serious complications associated with vitamin B12 deficiency including:

  • heart disease
  • neurocognitive disorders
  • coordination problems (ataxia)
  • peripheral neuropathy
  • vision loss
  • infertility (although this usually resolves with B12 treatment)
  • neural tube defects in the babies of women with B12 deficiency

High levels of assimilation by the body

B12 shots bypass the stomach and are absorbed directly into the bloodstream.

Therefore, they are a better option than oral supplements for those who have gastrointestinal issues, including older adults who have low levels of stomach acid or intrinsic factor.

Side effects and potential risks

Swollen ankles or feet are a potential side effect of vitamin B12 shots, and require immediate medical attention.

There is no upper limit for the intake of vitamin B12 because the risk of toxicity or overdose is extremely low.

However, B12 shots may have other side effects.

Mild side effects and potential risks, which should be referred to a doctor if they persist or worsen, include:

  • pain, redness, or itching at the site of the injection
  • mild diarrhea
  • swelling sensation in the body

More serious side effects, which require immediate medical attention, include:

  • muscle cramps
  • irregular heartbeat
  • unusual weakness or tiredness
  • swelling of the ankles or feet

Severe reactions are very rare but require emergency intervention. These include:

  • itching and swelling of the face, throat, or tongue
  • breathing difficulties
  • severe dizziness
  • sudden vision changes
  • slurred speech

Drug interactions

Vitamin B12 may interact with certain medications. People must always inform their doctor about all prescription and over-the-counter drugs they are taking before receiving a B12 shot.

Some commonly prescribed medications that may interact with B12 include:

  • Aspirin
  • antibiotics
  • H2 receptor antagonists
  • metformin
  • oral contraceptives
  • proton pump inhibitors

Those who have allergies or medical conditions should always inform their doctor before receiving a B12 shot.

Examples of allergies or conditions that may interact with B12 shots include:

  • allergy to cyanocobalamin or cobalt
  • hypokalemia (low potassium levels)
  • kidney disease
  • polycythemia vera (a rare blood disorder)
  • Leber’s disease (an eye disease)
  • deficiencies in other nutrients, particularly folic acid and iron

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