The body’s ability to absorb vitamin B12 from dietary supplements is largely limited by the capacity of intrinsic factor (a substance secreted by the stomach which enables the body to absorb vitamin B12. For example, only about 10 mcg of a 500 mcg oral supplement is actually absorbed in healthy people. (https://www.ncbi.nlm.nih.gov/pubmed/17991650?dopt=Abstract)
>We've verified successful B12 absorption post-inhalation in our own pilot study. Serum vitamin B12 levels were measured in the test subjects before inhalation of the aerosolized sample and 75 minutes after initial inhalation of the aerosolized sample. Before inhalation, serum B12 levels averaged 403.0 pg/mL. After inhalation, serum B12 levels averaged 424.7 pg/mL, a 5.4% increase compared to B12 levels before inhalation of the sample. This study demonstrated that the aerosolized vitamin B12 rapidly entered the bloodstream when inhaled by the test subjects.