Vitamin B12, also known as cobalamin, is an essential water-soluble B-complex vitamin that is necessary for the production of healthy red blood cells and in the maintenance of a healthy nervous system. Vitamin B12 is used to treat and prevent vitamin B12 deficiency, a condition in which vitamin B12 levels in the blood are low. Although vitamin B12 can be beneficial, a megadose has the potential to cause unwanted side effects.
Severe Allergic Reactions
Taking megadoses of vitamin B12 may cause severe allergic reactions in some patients. Patients allergic to cobalt and other ingredients in vitamin B12 should avoid taking this vitamin. Signs and symptoms of allergic reactions caused by vitamin B12 include wheezing, unusual hoarseness, problems breathing, chest tightness, chest pain, rashes, hives and facial swelling.
Blood Clots
Megadoses of vitamin B12 may cause blood clots in your extremities, according to Drugs.com. Blood clots are dangerous because they block the flow of blood to your heart and brain, leading to heart attack and stroke. Individuals at high risk for blood clots, such as cardiovascular patients, should consult the doctor before taking multivitamins containing large amounts of vitamin B12.
Hematologic Effects
Treatment of vitamin B12 deficiency using megadoses of the vitamin can cause polycythemia vera, which is a type of blood disorder characterized by an increase in blood volume and the number of red blood cells.
Optic Nerve Damage
Treatment of vitamin B12 deficiency with megadoses of vitamin B12 can harm your optic nerve and lead to blindness, according to Drugs.com. Individuals with a hereditary condition known as Leber's disease are at a greater risk for optic nerve damage. Patients with Leber's disease experience atrophy of the optic nerve, and taking vitamin B12 increases damage to the nerve.
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