Abnormally high levels of vitamin B12, or cobalamin, are unusual, according to MedlinePlus. However, there are a few side effects associated with taking vitamin B12, including headache, itching, edema, increased nervousness or anxiety, and involuntary or uncontrollable movements, according to Everyday Health.
More severe side effects of vitamin B12 range from acute allergic reactions to fluid in the lungs, decreased blood potassium levels, blood clots in the arms and legs, and congestive heart failure. Since extremely severe side effects may arise from using vitamin B12 as a therapy for megaloblastic anemia, patients should undertake this type of therapy under the stringent management of a doctor, as WebMD cautions. Also, vitamin B12 can severely damage the optic nerve, leading to blindness, which means that at-risk patients, such as those with Leber's disease, should not take B12 treatments.
Vitamin B12 is probably safe for most individuals as long as they take a dose that is under or within the recommended daily allowance, or RDA, asserts Mayo Clinic. The RDA of vitamin B12 differs, depending on an individual's age and stage of life, as the National Institutes of Health Office of Dietary Supplements explains. The RDA for vitamin B12 ranges from 0.4 micrograms for infants to 2.4 micrograms for adults, and it goes to 2.6 micrograms for pregnant women and 2.8 micrograms for nursing mothers.