Side Effects of Too Much Vitamin B12

Updated July 28, 2022
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Vitamin B12 is a water-soluble micronutrient found in foods such as meat, poultry, eggs, and dairy products. Your body needs this vitamin to make DNA and to keep your blood and nerve cells healthy. But taking too much vitamin B12 can lead to side effects like nausea and diarrhea.

If you are considering vitamin B12 supplementation, it is important to know the facts before you begin. Find out how much vitamin B12 you need and the best ways to get it naturally in your diet. Learning these basic facts can help you to make informed health decisions about the supplements you choose to take.

How Much Vitamin B12 Do I Need?

The National Institutes of Health (NIH) Office of Dietary Supplements provides recommended daily allowances (RDAs) for vitamins and minerals. The recommended amount of vitamin B12 varies according to your age range. Amounts are provided in micrograms (mcg).

  • Under 6 months: 0.4 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
  • 14-18 years: 2.4 mcg
  • 18 years and older: 2.4 mcg

Women who are pregnant and breastfeeding need the highest levels of vitamin B12. Pregnant women need 2.6 mcg each day and women who are breastfeeding need 2.8 mcg daily.

Can You Get Too Much B12?

According to the NIH, there is no tolerable upper intake level established for vitamin B12. The health organization also reports that there is no evidence that it can be harmful, even at high doses. But the vitamin can interact with medications and cause some side effects.

Medication Interactions

Vitamin B12 supplements can interact with certain medications, which is one reason you should always check with your doctor before taking high doses. St. Luke's Hospital says the following medications may interact with B12 in your body.

  • Anti-seizure medications
  • Bile acid sequestrants
  • Chemotherapy medications
  • Colchicine
  • H2 blockers
  • Metformin
  • Proton pump inhibitors

The organization notes that tetracycline and antibiotics can also interact with vitamin B12 if they are taken at the same time. They recommend taking the medications at a different time of day.

Elevated B12 in Cancer Patients

While vitamin B12 hasn't been shown to cause cancer, there is research showing a relationship. A 2016 study found that elevated B12 levels in cancer patients was associated with higher mortality rates than those with normal levels. An additional 2019 study found that folic acid and vitamin B12 supplementation may increase the risk of colon cancer. So again, always check with your doctor before beginning a B12 supplementation regimen.

Since a variety of factors are at play, it's best to get a doctor's recommendation if you are considering a vitamin B12 supplement or injection.

Vitamin B12 Side Effects

Vitamin B12 is generally well-tolerated, even in doses well above the recommended dietary allowance (RDA). Because vitamin B12 is a water-soluble vitamin and is excreted in the urine when consumed in excess, it's difficult to overdose on this essential nutrient. However, there are a few side effects to be on the lookout for if you're supplementing with vitamin B12 or receiving B12 injections. These side effects depend on the form of vitamin B12 you take.

Oral Vitamin B12

In healthy individuals, excess vitamin B12 likely won't cause problems. However, there are some side effects that may also be associated with B12 supplements when taken orally. These include:

  • Dermatological issues
  • Hand and feet tingling
  • Headache
  • Stomach issues (nausea, vomiting, diarrhea)
  • Tiredness, feeling weak

Vitamin B12 Injection

When receiving B12 injections due to B12 deficiency or inadequate absorption of vitamin B12, you might experience side effects related to the injection. According to The National Library of Medicine, common side effects include diarrhea and a swollen-feeling body, while less common (more severe) side effects may include:

  • Confusion
  • Coughing or wheezing
  • Difficulty breathing
  • Dizziness
  • Extreme thirst
  • Fast heartbeat
  • Fatigue
  • Frequent urination
  • Headache
  • High blood pressure after B12 injections
  • Hives, rash, and itching
  • Leg pain
  • Muscle pain or cramps
  • Red skin color
  • Shortness of breath
  • Swelling of arms and legs
  • Weakness

Always remember to communicate with your healthcare provider if you're experiencing any side effects from B12 injections.

B12 Nasal Gel

Taking B12 nasal gel for malabsorption (pernicious anemia) can cause some of the same side effects as B12 injections. According to MedlinePlus, unique side effects can include:

  • Bruising or unusual bleeding
  • Chills
  • Fever
  • Sore throat
  • Sore tongue
  • Stuffy nose

Talk with your healthcare provider if you experience side effects from B12 nasal gel.

How to Take Vitamin B12 Safely

Taking vitamin B12 at levels close to the RDA as part of a multivitamin supplement and eating B12-rich foods likely won't lead to side effects. Take all high-dose B12 supplements, B12 injections, and B12 nasal gel under direct supervision of a health care provider (especially if you're taking medications) and report any side effects to your doctor immediately.

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Side Effects of Too Much Vitamin B12