When taken at the suggested dosage, there are no side effects associated with B12 supplements, notes Drugs.com. For people who have a high number of red blood cells, also known as polycythemia vera, taking vitamin B12 can unmask the symptoms, according to WebMD.
Vitamin B12 creates new blood cells, and people who take chloramphenicol might not receive the benefits of taking a B12 supplement, states WebMD. People who have an allergy or sensitivity to cobalt or cobalamin must not take B12 supplements. Some people might have a sensitivity to B12 supplements used to treat eczema or psoriasis because they contain avocado oil.