Adverse Reactions to Vitamin B12 Shots

Written by contributing writer Google
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Vitamin B12 shots, which go by the generic name Cyanocobalamin and hydroxocobalamin often are administered to treat B12 deficiencies or as a supplement for patients who require higher levels of the vitamin due to a chronic disease. While there are rarely any side effects to taking oral doses of the vitamin, some people may suffer adverse reactions to B12 shots.

Mild Side Effects

Some relatively mild adverse reactions to vitamin B12 shots include temporary pain and a sensation of warmth at the site of the shot. The patient may also experience nervousness and anxiety. If these side effects persist, then the patient must seek medical attention form a doctor or pharmacist.

Benzyl Alcohol

When the B12 shot is administered to a newborn baby in the first few months of its life, the baby can have an adverse reaction to benzyl alcohol, a preservative that is contained in the vitamin B12 shot. Symptoms of an adverse reaction include a slow heartbeat, low blood pressure and sudden gasping for breath. The risk of an adverse reaction is even higher if the infant has a low birth weight, and it can sometimes lead to death. Ideally, the baby should be administered a B12 shot that does not use benzyl alcohol as a preservative.

Allergic Reaction

Some people may suffer adverse allergic reactions to the B12 shot, including difficulty breathing, chest pain or swelling of the tongue, throat, lips and face. These allergic reactions are infrequent, but can be serious. They occur because the different formulations of B12 shots, including hydroxycobalamin and cyanocobalamin, are synthetically derived formulations of the naturally-occurring vitamin B. In this case, the patient should instead take oral forms of B12 supplement, which typically do not cause allergic reactions.

Leber's Disease

Vitamin B12 shots may aggravate the progression of Leber's disease. Also referred to as Leber's Hereditary Optic Neuropathy (LHON), Leber's disease is a hereditary condition in which the individual's optic nerve gradually wastes away, progressively deteriorating the individual's vision. B12 shots can increase the rate of the nerve's atrophy, causing an even faster loss of vision.


B12 injections are especially used to administer high doses of the vitamin, commonly in amounts as high as 1,000mg. The National Institutes of Health report that high doses of B12 have been found to trigger a rare form of rosacea called rosacea conglobata. The report cite a case involving a 17-year old girl (See Resources section below.). The patient may also suffer a breakout of a skin rash or hives.

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