no it would not hurt him to do this but he needs to discuss this with the medical provider who is giving him the injections. b12 is typically used for pernicious anemia and sadly many well-meaning clinicians use it for other unproven reasons.
would it be ok for someone that gets b12 injections once a month to take b12 supplements occasionally? . my father gets b12 injections once a month due to b12 deficiency. usually by week 3 his symptoms of fatigue memory loss and irritability are prevalent. would it hurt for him to possibly take a b12 supplement in that last week before his scheduled injection?