When you take a supplement by mouth, you don’t get the full benefit. It’s normal to lose a significant amount of oral B12 supplements during the digestive process, which could mean you have much less B12 in your body than you think. But, with B12 injections, there’s no digestive process. The B12 goes right into your muscle, so you get its full benefits. This instant delivery also means you enjoy the benefits of B12 almost instantly while taking it by mouth means you’d have to wait for the lengthy digestive process to complete before feeling the effects of the B12.