Can Vitamin B-12 Cause Side Effects
You may need to take vitamin B-12 separately from the above drugs and supplements — say, one in the morning and one at night — so you can get the full dose of vitamin B-12.
Who needs vitamin B12 shots and why?
Vitamin B12 shots are injections that a doctor may prescribe to treat a vitamin B12 deficiency, especially if a person’s body has difficulty absorbing the vitamin.
A deficiency in vitamin B12 can lead to various health problems, ranging from fatigue to permanent neurological changes.
If a person has low vitamin B12 levels due to a health condition, a doctor may recommend oral supplementation or injections of the vitamin.
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 it having to go 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.
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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 start to 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
In animal-based foods, vitamin B12 binds to the protein molecules. During digestion, stomach acid causes it to separate from the protein, and a substance called intrinsic factor enables the bloodstream to absorb it.
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 who may needs shots include those who have had gastrointestinal surgery if their digestive system cannot absorb vitamin B12 efficiently.
The table below contains the recommended dietary allowances (RDAs) for vitamin B12. The RDA refers to the minimum daily amount that most healthy people belonging to 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 breastfeed | 2.8 mcg |
However, a doctor can advise on an individual’s specific needs.
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.
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.
That said, some people have a higher risk of deficiency and may benefit from vitamin B12 shots or tablets.
People with symptoms of vitamin B12 deficiency
Anyone who has 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
- dementia
- mood changes
- a sore tongue
- low appetite
- constipation
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.
- 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 surgery, may have fewer of the 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 aged over 60 years and that some people may benefit from vitamin B12 injections.
The scientists found that older adults have a higher risk of conditions linked to reduced stomach acid production, including gastric atrophy. Low stomach acid also enables certain bacteria to grow, and these can use up vitamin B12 stores.
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.
Research from 2010 looked at data from 689 men and found higher rates of vitamin B12 deficiency in those who consumed a plant-based diet. Over half of those who were vegan and 7% of those who were vegetarian had low vitamin B12 levels, compared with under 1% of people who ate meat.
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 breastfed, they may not consume enough vitamin B12. This can lead to permanent and severe neurological issues.
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.
Can Vitamin B-12 Cause Side Effects?
Your doctor will give you the right dosage of vitamin B-12 supplement, especially if you will inject it. Taking too much can cause side effects like nausea, headache, dizziness, and in rarely, blood clots or cancer.
Everyone needs vitamin B-12, and most people get enough through their diet. However, it’s important to know what side effects occur when you take too much.
Vitamin B-12 is water-soluble and absorbed in the intestines. After it’s absorbed, it’s used to make DNA and red blood cells. Vitamin B-12 that isn’t being used is stored in the liver. But if you take supplements, you might ingest more than your body needs.
Oral vitamin B-12 is safe to take at recommended doses for healthy people.
Injectable vitamin B-12, which is used to treat significant deficiencies, can cause the following side effects:
- mild diarrhea
- itching
- skin rash
- headache
- dizziness
- nausea
- vomiting
- pulmonary edema and congestive heart failure early in treatment
- vein thrombosis
- sensation of swelling
- polycythemia vera (rare, slow growing blood cancer)
Vitamin B-12 can cause very rare but serious allergic reactions (anaphylaxis). This includes swelling of the face, tongue, and throat, and difficulty swallowing and breathing. If this occurs after you take vitamin B-12, call 911 or your local emergency services immediately.
Some agents are linked to reduced absorption or serum levels of vitamin B-12. You may need vitamin B-12 supplements if you take any of the following:
- colchicine
- metformin
- proton pump inhibitors
- vitamin C
- chloramphenicol
- H2 blockers
- aminosalicylic acid
Folic acid supplements may interfere with vitamin B-12. Be sure to tell your doctor if you take folic acid.
You may need to take vitamin B-12 separately from the above drugs and supplements — say, one in the morning and one at night — so you can get the full dose of vitamin B-12.
Don’t take vitamin B-12 supplements if you have sensitivities or allergies to vitamin B-12, cobalt, and any other ingredients. Vitamin B-12 is safe to take in recommended dosages if you’re pregnant or breastfeeding.
If you notice any adverse effects after you start taking vitamin B-12 supplements, you should discontinue use immediately. Seek medical attention if your symptoms get worse or are severe.
You can consult your doctor to establish an appropriate dose if it’s determined that you don’t get enough B-12 from food sources.
According to the National Institutes of Health , the recommended dietary amounts (RDAs) are as follows:
- 2.4 micrograms (mcg) daily for ages 14 years or older
- 2.6 mcg daily for pregnant women
- 2.8 mcg daily for breastfeeding women
If you’re over the age of 50, you should try to get your RDA by eating foods with B-12 or by taking a supplement containing 25 to 100 mcg of B-12.
Foods with vitamin B-12 include:
See your doctor if you have bothersome side effects that don’t go away once you stop taking the supplements. Avoid taking supplements if possible, and try to get B-12 from food sources.
If you develop any side effects from taking vitamin B-12, you can stop taking the supplement and the symptoms should subside.
After this, you’ll need to determine either an appropriate dose or how to get the vitamin B-12 you need from food sources. You can discuss this with your doctor.
Last medically reviewed on August 9, 2019
How we reviewed this article:
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dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=a66eb3c4-3e1d-4d49-b963-4fa2334cc9b6 - Vitamin B-12. (2017).
mayoclinic.org/drugs-supplements-vitamin-b12/art-20363663 - Vitamin B12: Fact sheet for health professionals. (2018).
ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/
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