In medical circles, ‘oral absorption’ is an extremely common delivery method. Using nitroglycerin for heart patients, as a model example, the ONLY way that it is delivered is by ‘oral absorption’. Not only is it delivered by oral absorption, but over the last few years it has been increasingly delivered by spray (Medilingual®, from Rhone-Poulenc). If you were to take a nitroglycerin pill, no active nitroglycerin would ever enter the bloodstream, whereas a nitroglycerin spray, which is used in cases of angina pectoris, gets to work on the heart almost immediately.
Oral absorption also avoids degradation of an ingredients by stomach acids and other digestive juices.
Another benefit is that, due to the large amount of blood vessels in the mouth, oral absorption results in rapid onset of action and you can also use a lower dose because of the lack of destructive metabolism.
#VitaminB12 deserves special consideration. There is conclusive evidence, based on the medical literature, that vitamin B12 deficiency can be effectively treated using oral delivery as a convenient, easy, high compliance, even superior, alternative to injection.

Delpre, G. 1999. Sublingual Therapy for Cobalamin Deficiency as an Alternative to Oral and Parenteral Cobalamin Supplementation. The Lancet 354:740-741.

Sharabi, A., Cohen, E., Sulkes, J. and Garty, M. 2003. Replacement therapy for vitamin B12 deficiency: comparison between the sublingual and oral route. Br. J. Clin. Pharmacol. 56:635-8.

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