Vitamin B12 Shots: Uses, Benefits, And Side Effects

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Medical News TodaySubscribeWho needs vitamin B12 shots and why?

Medically reviewed by Megan Soliman, MD — Written by Jayne Leonard — Updated on January 26, 2024

What is vitamin B12?
What are vitamin B12 shots?
Who needs them?
How much do I need?
Benefits
Risks
FAQ
Summary

Vitamin B12 shots are injections that a doctor may prescribe to treat a vitamin B12 deficiency. They may be useful for those with gastrointestinal issues, older adults, vegans, and vegetarians.

A deficiency in vitamin B12 can lead to various health problems, ranging from fatigue to permanent neurological changes.

Vitamin B12 injections are usually for people with bodies that have problems absorbing vitamin B12 and those who have undergone gastric surgery. This is because shots enable the body to absorb vitamin B12 without going through the digestive system.

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

What is vitamin B12?

Glass Medicine Vials and Syringe — NikiLitov/Getty Images

Vitamin B12 is an essential water-soluble vitamin that plays a role in many bodily functions, including:

DNA production
red blood cells
nerve cells

Without vitamin B12, a person may feel tired and weak due to megaloblastic anemia.

Vitamin B12 is present in many dietary sources, such as:

meat
fish
eggs
dairy products
nutritional yeast
some fortified foods

Discover some of the best B12-containing foods here.

What are vitamin B12 shots?

Vitamin B12 shots are a form of supplementation that contains a synthetic version of vitamin B12, or cyanocobalamin. A doctor will inject the shot into the muscle. If they inject it into a vein, the body may lose a high proportion as the person passes urine.

Cyanocobalamin is available in liquid, tablet, and capsule form. Certain foods, such as cereals, may be fortified with the synthetic form of vitamin B12.

How long do vitamin B12 shots last?

How often a person will require shots depends on what is causing their B12 deficiency. Initially, a person may need B12 supplementation shots every day for around two weeks or until symptoms improve.

If the cause of a person’s B12 deficiency is their diet, they may not require further shots or only require them once or twice per year.

In cases where a chronic condition is causing a deficiency, people may require B12 shots around every 2 months.

Who needs vitamin B12 shots?

Vitamin B12 shots are only available by prescription following a clinical diagnosis of low levels. However, low levels are rare in most healthy adults because the human liver stores vitamin B12 over time.

Some people’s bodies do not produce enough stomach acid or intrinsic factor if they have a condition known as autoimmune atrophic gastritis. These people may need vitamin B12 shots to reduce their risk of deficiency, which can lead to pernicious anemia.

Other people needing shots include those who have had gastrointestinal surgery if their digestive system cannot absorb vitamin B12 efficiently.

People with symptoms of vitamin B12 deficiency

Anyone with signs and symptoms of a vitamin B12 deficiency or pernicious anemia should consult a doctor.

Signs and symptoms include:

difficulty thinking and remembering
fatigue
heart palpitations
pale skin
weight loss
infertility
numbness and tingling in the hands and feet

Vitamin B12 deficiency risk factors

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

high alcohol consumption
older age
pernicious anemia
atrophic gastritis, which refers to inflammation in the stomach
Helicobacter pylori infection
celiac disease
Crohn’s disease
a history of gastrointestinal surgery
following a plant-based diet
pancreatic insufficiency
AIDS
some hereditary conditions that affect vitamin B12 absorption

People with gastrointestinal issues

Conditions that affect the gastrointestinal tract may affect vitamin B12 release or absorption.

These include:

pernicious anemia, which can lead to gastric atrophy, or damage to the stomach
fish tapeworm infestation
bowel or pancreatic cancer
folic acid deficiency
overgrowth of bacteria in the small intestine
celiac disease
Crohn’s disease

Individuals who have undergone gastrointestinal surgery, including weight loss, may have fewer cells necessary to secrete stomach acid and intrinsic factor. This can also affect vitamin B12 absorption.

Older adults

Research from 2015 suggests that vitamin B12 deficiency is more likely to affect those over 60 years and that some people may benefit from vitamin B12 injections.

Vegetarians and vegans

Vitamin B12 mainly occurs in animal foods, so people who follow a plant-based diet have a higher risk of vitamin B12 deficiency.

In rare cases, a doctor may recommend injections, but research suggests that taking additional vitamin B12 by mouth can be as effective as receiving an injection in a muscle. It is also less costly.

Vegetarian or vegan people who are pregnant may need to take supplements or consume fortified foods, as vitamin B12 transfers to the baby via the placenta and breast milk. If the infant is entirely chestfed, they may not consume enough vitamin B12. This can lead to permanent and severe neurological issues.

Discover the best vegan and vegetarian sources of B12 here.

How much vitamin B12 do I need?

The table below contains the recommended dietary allowances (RDAs) for vitamin B12. The RDA refers to the minimum daily amount that most healthy people in a specific group need.

Group	Amount
0–6 months	0.4 micrograms (mcg)
7–12 months	0.5 mcg
1–3 years	0.9 mcg
4–8 years	1.2 mcg
9–13 years	1.8 mcg
14+ years	2.4 mcg
Pregnant people	2.6 mcg
People who chestfeed	2.8 mcg

However, a doctor can advise on an individual’s specific needs.

Benefits

A doctor may recommend vitamin B12 shots for people at risk of a deficiency or its complications.

Vitamin B12 shots may help reduce the risk of the following conditions:

heart disease
stroke
neurological disorders
problems with thinking and memory
vision loss
infertility
neural tube defects in children born to those with a vitamin B12 deficiency

Risks

There is no upper limit for the intake of vitamin B12 because the risk of toxicity or overdose is low. However, vitamin B12 shots may have other side effects.

After having a B12 shot, a person should monitor for adverse symptoms and seek medical help if they experience:

severe pain at the injection site
mild diarrhea
a swelling sensation in the body
temporary itching of the skin

In rare cases, a person may also be at risk of:

anaphylaxis
pulmonary edema
congestive heart failure
peripheral vascular thrombosis, which involves blood clots
polycythemia vera, which is a type of blood cancer

If anyone experiences difficulty breathing, hives, and swelling, they need emergency medical help. They may be experiencing anaphylaxis, which is a severe allergic reaction.

Medications side effects

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

Some other prescription medications can directly cause a vitamin B12 deficiency. These include:

H2 receptor antagonists
metformin
proton pump inhibitors

Allergies and medical conditions

Those with allergies or medical conditions should always inform a doctor before receiving a vitamin B12 shot. Vitamin B12 shots may not be suitable for people with a history of:

sensitivity to vitamin B12
Leber’s disease, which affects the optic nerve
kidney problems
hypokalemia, or low potassium levels
deficiencies in other nutrients, particularly folic acid and iron

Frequently asked questions

Here are some frequently asked questions about vitamin B12 injections.

What does a B12 shot do for you?

Vitamin B12 injections provide the individual with a synthetic version of vitamin B12. A doctor may recommend vitamin B12 shots if a person has a vitamin B12 deficiency.

Do vitamin B12 injections work immediately?

Vitamin B12 injections typically begin to work straight away. However, it may take a number of days or weeks before a person notices improvements in their symptoms.

What should you not do after a B12 shot?

Precautions a person may need to take after receiving a B12 injection may depend on which side effects they experience. For example, if they experience dizziness, they should avoid driving. A person’s doctor can provide them with information about any specific activities they may need to avoid or any other considerations. This might include limiting alcohol intake.

Summary

Most people get enough vitamin B12 from their diet, but some people have a deficiency. This can be due to having low levels of intrinsic factor in the digestive system, having a digestive condition, or following a plant-based diet.

If dietary sources are insufficient, however, a doctor may recommend supplementation through pills or injections, depending on the cause of the deficiency.

Nutrition / Diet

How we reviewed this article:

SourcesMedical News Today has strict sourcing guidelines and relies on peer-reviewed studies, academic research institutions, and medical journals and associations. We only use quality, credible sources to ensure content accuracy and integrity. You can learn more about how we ensure our content is accurate and current by reading our editorial policy.

Ankar A, et al. (2021). Vitamin B12 deficiency.https://www.ncbi.nlm.nih.gov/books/NBK441923/
Autoimmune atrophic gastritis. (2016).https://rarediseases.info.nih.gov/diseases/10310/autoimmune-atrophic-gastritis
Common questions about hydroxocobalamin. (2022).https://www.nhs.uk/medicines/hydroxocobalamin/common-questions-about-hydroxocobalamin/
Higdon J, et al. (2015). Vitamin B12.http://lpi.oregonstate.edu/mic/vitamins/vitamin-B12
Label: Vitamin deficiency injectable system - B12 - cyanocobalamin kit. (2020).https://dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=579bd1fe-51a1-403f-a422-0fb701aab57d
Miller JW. (2018). Proton pump inhibitors, H2-receptor antagonists, metformin, and vitamin B-12 deficiency: clinical implications.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6054240/
Recommended dietary allowance. (n.d.).https://www.sciencedirect.com/topics/food-science/recommended-dietary-allowance
Vitamin B12: Fact sheet for health professionals. (2021).https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/
VItamin B12 or folate deficiency anaemia. (2023).https://www.nhs.uk/conditions/vitamin-b12-or-folate-deficiency-anaemia/
Wang H, et al. (2018). Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6494183/
Wong CW, (2015). Vitamin B12 deficiency in the elderly: Is it worth screening?.https://pubmed.ncbi.nlm.nih.gov/25756278/

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Medically reviewed by Megan Soliman, MD — Written by Jayne Leonard — Updated on January 26, 2024

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