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What Are Side Effects of Vitamin B12 Tablets?

author image Matthew Busse
Matthew Busse has pursued professional health and science writing since 2007, writing for national publications including "Science Magazine," "New Scientist" and "The Scientist." Busse holds a doctorate in molecular biology from the University of California-San Diego.
What Are Side Effects of Vitamin B12 Tablets?
Close-up of B12 supplements. Photo Credit Scharvik/iStock/Getty Images

Vitamin B12, also known as cobalamin, is one of the largest and most complex vitamins. Several biological processes require vitamin B12, and deficiencies of this vitamin can cause several health conditions, including anemia. While vitamin B12 is generally safe, some side effects may occur. As with any health supplement, you should consult your doctor before taking vitamin B12.


Vitamin B12 is a water-soluble vitamin, meaning if you take too much, the excess amounts are easily flushed from the body through urine. As a result, taking large doses of vitamin B12 rarely results in toxicity. Current research suggests that you can safely take up to 1 milligram of vitamin B12 per day and experience no major side effects, reports the Linus Pauling Institute at Oregon State University.

Allergic Reaction

In rare cases, certain individuals may experience a severe allergic reaction to vitamin B12, which contains a metal ion, cobalt, and anyone allergic to cobalt will also be allergic to vitamin B12. Symptoms of an allergic reaction include swelling of the hands, face, neck, mouth or tongue. Dizziness, difficulty swallowing or breathing and unconsciousness can occur during serious allergic reactions. If you experience these symptoms after taking vitamin B12, contact your doctor immediately.

Additional Side Effects

Some people might experience a variety of mild side effects after taking vitamin B12, such as temporary itching, rash or rosacea fulminans, an intense reddening of the face. Diarrhea also has been reported, Certain individuals may develop an increased risk of blood clots, particularly if vitamin B12 supplements are taken to treat vitamin B12 deficiency. Treatment of a B12 deficiency can unmask a blood disorder known as polycythemia vera, which is characterized by increased red blood cells and blood volume.


People with a rare, hereditary optic nerve disease known as Leber's disease should not take vitamin B12 supplements. Doing so may result in a rapid, severe atrophy of the optic nerve. People who have recently received a coronary stent though angioplasty should avoid vitamin B12. After the placement of coronary stents, patients often receive high doses of vitamin B6 or B12, and taking additional vitamin B12 can reduce the effectiveness of the stents and cause a re-narrowing of the arteries.

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