Vitamin B-12, or cyanocobalamin, is a water-soluble vitamin that plays an important role in metabolism. It helps in the formation of the red blood cells and promotes functioning of the central nervous system. Deficiency of this vitamin can lead to enlarged red blood cells, loss of appetite and constipation, along with memory loss, disorientation and mood changes. To avoid the deficiency, vitamin B-12 can be obtained from dietary sources such as meat, eggs and milk. Strict vegetarians and vegans may need vitamin B-12 supplements. The supplements, however, are associated with a variety of side effects.
Heart Problems
Vitamin B-12 can be given orally, nasally or as an injection, and the route depends on the severity of the deficiency. Vitamin B-12 injections and supplements may lead to peripheral vascular disorders in a small number of cases, according to the website Drugs.com. This can affect the flow of blood via veins and arteries and increase the risk of heart attack and stroke. It can also lead to the formation of clots in the blood vessels and abnormal accumulation of fluids in the lungs. These side effects usually occur early in treatment. Tell your doctor right away if you suspect you have any of these conditions.
Skin Reactions
Drugs.com also reports that about 8 percent of those who receive vitamin B-12 via nasal administration develop a rash. Injections of the vitamin can also lead to itching and acne. Intense erythema, or redness of the skin may require topical or systemic corticosteroid therapy, according to the Mayo Clinic website.
Vision Loss
Immediate damage to the optic nerve may occur in individuals suffering from a rare hereditary disorder known as Leber's disease, which results from mutations in the mitochondrial DNA of the cells, warns the National Institutes of Health. This can lead to severe complications, including blindness. Discuss pre-existing conditions with a doctor before taking vitamin B-12 supplements.
Pain
Both intranasal and injection administration of vitamin B-12 can lead to back pain, joint pain and muscle pain, says Drugs.com. Consult a doctor before taking pain medications to treat these conditions to avoid complications.
Polycythemia Vera
Polycythemia vera is an abnormal increase in the concentration of red blood cells and is characterized by breathing difficulties, dizziness, and headache. The National Institutes of Health says that polycythemia vera may be associated with vitamin B-12 usage. In fact, an elevated vitamin B-12 level is seen in 30 to 50 percent of those with polycythemia vera, according to an article published in the November 2003 edition of the journal "Clinical Biochemistry."
Hypersensitivity
Hypersensitivity or allergic reactions may occur in some rare cases. Allergic reactions are more common when vitamin B-12 is administered via injection. These reactions are commonly characterized by rash, gastric disturbances, stuffy nose and headache. More severe anaphylactic reactions and death are extremely rare. But contact a doctor immediately if any hypersensitivity signs appear, as they may life threatening.
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