Some side effects of vitamin B12 include clumsiness, dizziness, anxiety, a reduced sense of touch and weakness, according to Drugs.com. People can also suffer from itching and rash and a worsening of inflammatory acne. Other side effects are nausea, vomiting and diarrhea that usually goes away on its own.
Some people have also suffered from pulmonary edema, blood clots in their peripheral blood vessels and congestive heart failure, notes Drugs.com. Other patients have experienced muscle and back pain, arthritis, shortness of breath and runny nose. People have also suffered overall swelling and pain and infection because of a compromised immune system.
Vitamin B12 side effects include an abnormally large number of red blood cells, a condition known as polycythemia vera. The vitamin has also been known to unmask the symptoms of this disorder, explains WebMD. These symptoms include itchiness, headache, dizziness, joint swelling, shortness of breath and weakness. Polycythemia vera is a type of slow growing blood cancer, according to the Mayo Clinic.
Vitamin B12 can cause serious side effects in people with megaloblastic anemia and Leber's disease, states WebMD. It should not be taken by people who are allergic to cobalt or cobalamin or taken in combination with folate and vitamin B6 after receiving a coronary stent.