Transdermal routes often struggle with penetration in a predictable way
However, a very small percentage of B12 from a supplemental dose is absorbed by passive diffusion, rather than by intrinsic factor, such that vegans can make up for taking B12 only once per day, or even only once per week, by taking larger doses than normally provided by food
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Injectable Peptides Most peptides are poorly absorbed when taken orally because digestive enzymes and stomach acid break them down before they can reach the bloodstream
doi: 10.1016/J.IJBIOMAC.2024.129651 48 El-FawalG
For most people, the safest approach is to compare a few reputable suppliers rather than buying from the first website they find