How often do you obtain B12 injections for weight loss? How often should vitamin B12 injections be administered and how frequently should laboratory monitoring be performed? How Long Does It Take Vitamin B12 Injections to Work? How Often Can You Get a B12 Shot for Maximum Health? Gym goers spending monthly on vitamin B and C injections Have a f*cking word with yourself. Both are water soluble, so don't stick around for long in your system (12 14hrs) hence