Nervous System Reactions. After getting a vitamin B12 injection some people may experience a disturbance to the nervous system. These symptoms may include dizziness, difficulty walking normally, nervousness, anxiety, weakness and uncoordinated movements.lthough it is a rare occurrence, an allergic reaction may occur following a vitamin B12 injection. The reaction is thought to occur due to impurities in the B12 injection, notes Drugs.com.