Some uninformed practitioners believe that it is not possible to overdose on Vitamin B12, however 5-10% of the population present with above-normal levels of Vitamin B12, so if injections are erroneously given, they can trigger a number of adverse symptoms that may include coronary artery spasms with chest pains, numbness or pain down their right arm, and or TIA-like symptoms with tingling numbness on the right side of the face. There is also a risk of hypokalemia, pulmonary edema, peripheral vascular thrombosis, optic nerve atrophy for someone with Leber's disease, and others. Risks from intranasal use of Vitamin B12 include glossitis, headaches, sore throat, rhinitis, and feeling of pins and needles. Individuals suffering from Mitral Valve Prolapse are also vulnerable to experience a worsening, or an acute flare-up of MVP symptoms following a Vitamin B12 shot if not indicated... Full Text