Side Effects of Too Much Vitamin B12

Reviewed by Terri Forehand RN
B vitamins

Getting too much of a good thing can cause unpleasant, even dangerous, side effects. This is true for many essential vitamins and minerals, including vitamin B12. Knowing the facts before you begin vitamin B12 supplementation helps you make informed health decisions and determine if B12 supplements or injections are the right fit for you.

Potential Side Effects

Vitamin B12 is generally well-tolerated, even in doses well above the recommended dietary allowance (RDA), which is 2.4 micrograms daily for adult men and women. 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.

Optic Nerve Damage

In healthy individuals, excess vitamin B12 likely won't cause problems. However, the University of Maryland Medical Center says if you have an eye disease called Leber's disease, B12 supplements can seriously damage your optic nerve. So always check with your doctor first, especially if you have pre-existing health conditions like Leber's disease before taking B12 supplements of any kind.

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. The University of Maryland Medical Center says the following medications may interact with B12 in your body:

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

Mayo Clinic says the following medications may also interact with B12 in your body:

  • ACE Inhibitors
  • Bone loss medications
  • Cancer medications
  • Gout medications
  • High blood pressure medicines
  • Nervous system medications
  • Alcohol
  • Aspirin
  • Birth control pills
  • Nicotine
  • Nitric oxide
  • Stimulants
  • Heart medications

Cancer Risks

While vitamin B12 hasn't been shown to cause cancer, Mayo Clinic suggests using B12 supplements with caution if you have a history of cancer. While B12's relationship to cancer requires further research, a 2013 study published in the Journal of the National Cancer Institute found cancer risks increased with higher B12 blood levels. So again, always check with your doctor before beginning a B12 supplementation regimen.

Other Potential Side Effects

According to Mayo Clinic, the following side effects may also be associated with B12 supplements or injections:

  • Narrowing of blood vessels in patients after stent placements
  • High blood pressure after B12 injections
  • Rash, itching, and burning of the skin
  • Pink or red skin discoloration
  • Facial flushing
  • Urine discoloration
  • Nausea
  • Difficulty swallowing
  • Diarrhea
  • Increased blood volume
  • Increases in red blood cells
  • Low potassium levels
  • Elevated uric acid levels, or gout attacks in people with a history of gout

B12 Injection Side Effects

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 MedlinePlus, common side effects include diarrhea and a swollen-feeling body, while less common (more severe) side effects may include muscle pain or cramps, leg pain, weakness, extreme thirst, frequent urination, confusion, shortness of breath, coughing, wheezing, fatigue, fast heartbeat, swelling of arms and legs, headache, dizziness, red skin color, hives, rash, itching, and difficulty breathing. Talk to your doctor if you're experiencing any such side effects from B12 injections.

B12 Nasal Gel Side Effects

Taking B12 nasal gel for malabsorption (pernicious anemia) can cause some of the same side effects as B12 injections. According to MedlinePlus, these may include headache, upset stomach, stuffy nose, sore tongue, weakness, bruising or unusual bleeding, muscle weakness, cramps, muscle pain, leg pain, extreme thirst, frequent urination, confusion, burning or tingling in hands and feet, sore throat, fever, chills, rash, hives, itching, and difficulty breathing. Talk with your doctor if you experience side effects from B12 nasal gel.

Avoiding Side Effects

Though side effects of excess B12 are rare, there are several things you can do to avoid effects of high doses of vitamin B12 supplements and injections. Eat plenty of vitamin B12-rich foods (no side effects occur from doing this) and take a multivitamin supplement containing B12 in amounts comparable to RDAs, which include the following daily dosages:

  • Children ages 1 to 3: 0.9 micrograms
  • Children ages 4 to 8: 1.2 micrograms
  • Kids ages 9 to 13: 1.8 micrograms
  • Adult men and women: 2.4 micrograms
  • Pregnant women: 2.6 micrograms
  • Breastfeeding women: 2.8 micrograms

How Much Is Too Much?

Because there isn't a tolerable upper intake level established for vitamin B12, it's difficult to determine how much is too much. However, Mayo Clinic says that doses of up to 2,000 micrograms from B12 supplements have been safely taken daily for up to 2.5 years in people with vitamin B12 deficiencies. Because a variety of factors are at play (medication interactions, B12 absorption rates in your body, etc.), it's best to get a doctor's recommendation for individualized B12 supplement or injection needs, especially if you have pernicious anemia.

Taking 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.

Side Effects of Too Much Vitamin B12