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Too much B12 symptoms: What are the early signs of too much B12?

Medically reviewed by Karen Berger, Pharm.D.
Licensed Pharmacist
Updated Jun 10, 2024  •  Published Jan 10, 2024
Datos Verificados
In this article:

Overview: What does too much B12 feel like?

Vitamin B12 is a water-soluble vitamin necessary for nervous system function, red blood cell production, and cell division. Most people get all the vitamin B12 the body needs from a healthy diet. Some people, such as older adults, pregnant women, breastfeeding women, people on a vegan or vegetarian diet, or people taking metformin or a proton pump inhibitor, may need to take over-the-counter or prescription supplements to get enough vitamin B12. These supplements usually contain cyanocobalamin. People taking vitamin B12 supplements often take other supplements, such as vitamin B9 (folate or folic acid) and vitamin B6 (pyridoxine). People with a serious B12 deficiency may need B12 intramuscular injections.

While other vitamins and minerals may be toxic at high doses, vitamin B12 is considered safe even at high doses. Excess vitamin B12 is eliminated in the urine, so taking too much B12 is simply spending money to flush it down the toilet. However, taking prescription levels of B12 doses may cause mild symptoms in some people. On the other hand, B12 shots commonly produce side effects such as nausea, headache, and other problems. 

RELATED: 9 common nutrient deficiencies in the U.S.

Key takeaways:

  • Too much B12 is probably a common health condition that mostly affects people taking over-the-counter or prescription vitamin B12 supplements. 

  • Most people taking too much B12 will probably not experience symptoms. Early signs of too much B12 include mild diarrhea and itching.

  • Too much B12 is not associated with serious symptoms, but vitamin B12 injections can cause serious side effects that may require emergency medical treatment.

  • Too much B12 is caused by vitamin B12 injections or high doses of B12 oral supplements. You may be at risk for developing symptoms if you are receiving vitamin B12 injections to address a B12 deficiency.

  • Too much B12 usually requires a medical diagnosis, but it is not considered a serious problem if it’s due to taking too many supplements.

  • Too much B12 generally does not require treatment other than stopping or reducing the supplementation. Symptoms of too much B12 typically resolve in a day or two if oral supplements or injections are stopped or reduced. 

  • Treatment of too much B12 may include stopping the supplement, symptom relief medications, and supportive care if symptoms are severe.

  • Untreated and persistently high levels of B12 could result in complications like an increased risk of death in older adults. Vitamin B12 injections can cause serious side effects such as low potassium, heart failure, blood clots, or fluid buildup in the lungs.

  • Save on prescriptions for too much B12 with a SingleCare prescription discount card.

What are the early signs of too much B12?

Most people who take too much vitamin B12 do not have symptoms. Early signs of too much B12 are more likely in people taking prescription-level daily doses of 1,000 to 2,000 micrograms or more. Even then, side effects are unlikely. At prescription-strength doses, only about 1% of the vitamin B12 is absorbed into the bloodstream. Excess B12 in the blood is rapidly excreted through the urine. People are most likely to experience symptoms if they are being given B12 injections.

Possible early signs of too much B12 include:

  • Mild diarrhea

  • Itching

Possible early signs of too much B12 injections include:

  • Pain

  • Headache

  • Mild diarrhea

  • Nausea

Other symptoms of too much B12

For people who overdo oral vitamin B12 supplements, symptoms are unlikely. The body’s gastrointestinal system has an upper limit on the amount of B12 it can absorb. However, there are rare reports of symptoms in people taking prescription-level oral B12 at doses of 1,000 to 2,000 mcg. These include:

  • Mild diarrhea

  • Itching

  • Acne or rosacea

For people taking megadoses of vitamin B12 supplements, severe symptoms are very rare and include:

  • Heart palpitations

  • Restlessness

  • Anxiety

  • Insomnia

  • Facial redness

Vitamin B12 shots are more likely to cause problems. These include:

  • Pain

  • Headache

  • Itching

  • Skin rash

  • Fever

  • Shortness of breath

  • Rapid weight gain

Other possible severe side effects of injectable vitamin B12 include:

RELATED: How to make sure you absorb your supplements

Too much vitamin B12 vs. vitamin B12 deficiency

Both too much vitamin B12 and too little vitamin B12 may have no symptoms. It’s hard to get too much vitamin B12 because it’s poorly absorbed at high doses, and the body quickly eliminates it. That means that high B12 levels don’t last very long. Some people with low levels of vitamin B12 may have no symptoms, or the symptoms may be subtle. When they do, many of the symptoms are similar to a vitamin B12 excess. The critical difference is this: a vitamin B12 deficiency primarily causes anemia, a shortage of red blood cells. The symptoms of anemia, such as fatigue, pale skin, and yellowing of the skin, are critical clues in the diagnosis of B12 deficiency.

Too much vitamin B12 Too little vitamin B12
Shared symptoms
  • No symptoms
  • Headache
  • Diarrhea
  • Fatigue
Unique symptoms
  • Nausea
  • Vomiting
  • Acne or rosacea
  • Pale skin
  • Yellowing of the skin
  • Swollen tongue
  • Red patches or lesions on the tongue
  • Numbness or tingling
  • Burning sensation in the mouth
  • Slow reflexes
  • Problems walking
  • Loss of balance or coordination
  • Eye problems
  • Mood changes
  • Memory problems
  • Alterations in mental status

RELATED: Vitamin B12 deficiency symptoms: what are the early signs of vitamin B12 deficiency?

When to see a doctor for too much B12 symptoms

Too much B12 does not commonly cause symptoms. When it does, they tend to be mild and not easily identified with any specific cause. 

If you are self-administering B12 supplements and have mild problems like headaches or tiredness, then consider cutting back your intake to the recommended daily allowance. If that doesn’t work, consider discontinuing the supplement. 

B12 injection side effects, however, are a different matter. They can cause serious side effects. When receiving injections, always report side effects to the healthcare professional overseeing treatment. 

Diagnosing too much B12 is not a critical task since the body clears B12 rapidly. If a clinician suspects that too much B12 is an issue, you will probably be asked to quit taking supplements. Diagnosing B12 injection side effects is a critical task, particularly since some of the worst side effects, like heart failure, can occur early in treatment. For this reason, people receiving B12 shots are given regular blood tests to monitor B12 levels and other critical substances such as potassium and homocysteine.

Complications of too much B12

There are no complications associated with untreated high levels of B12 in the body. However, one study found that persistently high levels of vitamin B12 in older adults increased the risk of death from all causes independent of other risk factors. High vitamin B12 levels are also associated with a higher mortality rate in people on hemodialysis.

How to treat symptoms of too much B12

The standard treatment for too much B12 is to stop taking supplements or to reduce injection doses. There is no antidote if too much B12 is taken or injected, but B12 is not considered toxic or life-threatening. If too much B12 produces symptoms, the only options are symptom relief treatments and supportive care. 

Signs too much B12 symptoms are getting better

If B12 levels are high, they quickly return to normal after stopping or reducing supplements. As B12 levels return to normal, any symptoms will improve.

For normally healthy children and adults, it’s usually not necessary to take excess vitamin B12. At best, high-dose B12 supplements are a waste of money. Why? Most of the active ingredient in the supplement ends up in the toilet. Instead, take the recommended daily allowance (RDA) without dramatically overdoing it. In adults, the body needs only 2.4 mcg daily. Pregnant women need 2.6 mcg daily. Breastfeeding women need 2.8 mcg per day. Large doses and injections are only needed in people at risk for vitamin B12 deficiency or who have been diagnosed with vitamin B12 deficiency. 

For vitamin B12 the best way to get enough vitamin B12 is to eat a balanced diet providing all the essential nutrients. The National Institutes of Health Office of Dietary Supplements has a useful list of B12 food sources, including animal products, nutritional yeast, seafood, dairy products, breads, and breakfast cereals. If you’re worried about vitamin B12 intake, then consult this list to change your diet or get advice from a nutritionist or healthcare professional. If you’re considering taking B12 or other vitamin supplements, get medical advice first. 

RELATED: What vitamins should I take?

What’s next? Additional resources for people with symptoms of too much B12

Test and diagnostics

Treatments

Scientific studies and clinical trials

Medically reviewed by Karen Berger, Pharm.D.
Licensed Pharmacist

After receiving her doctorate from the University of Pittsburgh School of Pharmacy, Karen Berger, Pharm.D., has worked in both chain and independent community pharmacies. She currently works at an independent pharmacy in New Jersey. Dr. Berger enjoys helping patients understand medical conditions and medications—both in person as a pharmacist, and online as a medical writer and reviewer.

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