Oral vitamins are poorly absorbed. Most of over the counter formulations use cheap materials (inactive forms) that have a very low bioavailability. There are not a lot of profit margins in vitamins as compared to pharmaceuticals. Bioavailability depends on quality of formulation, quantity of vitamin, proper digestion, normal gut health, alcohol, caffeine and stress to name a few. Furthermore, fillers, coatings, excipients, and surfactants of products affect the completeness or rate of release of a particular vitamin.