Vitamin B12 is a water soluble vitamin that the body needs for making DNA and for healthy blood and nerve cells. According to eMedTV, vitamin B12 is also one of the vitamins needed to break down the amino acid homocysteine. The body obtains its vitamin B12 through digestion of animal protein. Vitamin B12 is then absorbed through the small intestine. Some people cannot absorb enough B12 in the intestine and need to take supplements. Supplements can be taken by oral route, in a patch or as an injection or nasal spray. Side effects are rare but may occur.
Allergic Reaction
It is possible to have an allergic reaction to vitamin B12 or the manufactured forms called cyanocobalamin and hydroxocobalamin, according to MayoClinic.com. Signs of an allergic reaction include hives, difficulty swallowing, wheezing and swelling of the face, mouth and throat. Any sign of an allergic reaction should be reported to a physician at once.
Blood Clots and Swelling
Vitamin B12 is considered safe if taken at recommended dosages. According to Oregon State University, the recommended daily allowance for vitamin B12 for adults is 2.4 to 2.8 micrograms. Another side effect of taking supplemental B12 can be the development of clots in the legs. It is also possible to develop a feeling swelling all over the body, according to eMedTV. Both of these side effects are uncommon and typically associated with injections of B12. The development of either of these side effects should be reported to a physician immediately.
Nausea, Diarrhea and Vomiting
Those taking supplemental vitamin B12 may experience nausea, vomiting and transient diarrhea. These side effects may appear when first taking vitamin B12 and then clear up as the body becomes used to it. To avoid these gastrointestinal effects, if taking vitamin B12 via the oral route, it may help to take the supplement on a full stomach. Vitamin B12 should be taken under a physician's guidance and only taken as directed. Drugs.com states that when a dose is missed, the regular dose should be taken at its next usual time without doubling the dose.
Itching and Skin Rash
There are some dermatological side effects associated with taking vitamin B12 supplements, including the development of itching and a skin rash. According to Drugs.com, when taking vitamin B12 by injection or as a nasal spray, some have developed symptoms of acne and folliculitis, a mild skin infection, at doses of 5 to 10 mcg per week.
Drug Interactions
Vitamin B12 can interact with some medications causing less vitamin B12 to be absorbed. According to the Office of Dietary Supplements, vitamin B12 can react with omeprazole, lansoprazole, cimetidine, ranitidine and famotidine, which are used to treat peptic ulcer disease and acid reflux, the antibiotic chloramphenicol and mertformin, a drug used to treat diabetes. Vitamin B12 supplements should be taken under a physician's supervision, and the physician managing the vitamin B12 supplement should be informed of all other medications being taken.
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