Can Vitamin B-12 Cause Side Effects?

Curated by Claudia Shannon / Research Scientist / ishonest

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.

What are the side effects of vitamin B-12?

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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
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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.

When should you see a doctor?

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.

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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.

What’s the correct dosage of vitamin B-12?

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:

  • cheese
  • fish
  • shellfish
  • liver
  • kidney
  • red meat

How are vitamin B-12 side effects treated?

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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.

The bottom line

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.

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