What are the adverse effects of methylcobalamine injection 500 mcg on alternate days for upto 7 injections, can the first dose cause anaphylaxis?

Need 2nd dose. Usually, anaphylaxis (& allergic reactions) requires a SECOND exposure to the foreign substance. The first exposure, which could be decades ago, "primes" the immune system. The second & subsequent exposures cause anaphylaxis. However, methycobalamine (Vitamin B12) is a natural & necessary vitamin, so if u react, it wd b from a preservatives or excipient in the injection. There r few other side eff.