Administering the vitmains VIA injection has a much higher bioavailablity than its oral counterpart. The last time I checked a couple months ago, from what I remember (again regering to b12, because it has near identical pharmacology) the bioavailability of oral b12 was around 60-70% at the usual dietary intake levels. This number decreased by near 50% once the dosages were raised over 1g (oral). So administering the vitamin straight into the body (injecting), bypasing digestive restrictions, you would yield a contstant 90% + bioavailability, and due to the fact its water soluble it would hold the same halflife of 9-12 hours before elimination.