What are the side effects of vitamin B12?

Vitamin B-12 side effects are rare when taken in correct doses, but high doses may cause acne, states WebMD. Severe allergic reactions to the main ingredient in vitamin B-12 supplements, cyanocobalamin, can cause chest tightening, breathing problems, hives and swelling of the face, tongue, mouth and lips, notes Drugs.com.

People who take chloramphenicol long term should speak to a doctor about their B-12 levels, as chloramphenicol may decrease the effects of B-12 on new blood cells. Medications for acid reflux or diabetes may cause the body to absorb vitamin B-12 poorly. Some people put vitamin B-12 mixed with avocado oil directly on their skin to treat psoriasis or eczema. Mild itching has been reported as a side effect by one person as of 2015, states WebMD.

Women who are pregnant or breastfeeding should not exceed their doctor's recommended daily dose of vitamin B-12, as too much may be dangerous. People with a high red blood cell count should speak to a doctor before taking vitamin B-12. People with a sensitivity or allergy to cobalt or cobalamin should not take it. Those with Leber's disease should not take B-12, as it can cause blindness, according to WebMD.

Vitamin B-12 is taken to prevent or to treat a B-12 deficiency, notes WebMD. People also take vitamin B-12 supplements to boost their energy, to prevent or treat Alzheimer's disease, to boost their mood, and to treat sleep disorders. Vitamin B-12 may also be taken for depression, diabetes, male infertility, AIDS, inflammatory bowel disease and swollen tendons.