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:

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

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.