This is the largest study that documents therapy with Sublingual preparations of Vitamin b12 to be sufficient and even superior to the intramuscular administration route. The sublingual overcomes the challenges of IM injections and should be the first line option for patients with Vitamin B12 deficiency.
https://pubmed.ncbi.nlm.nih.gov/30632091/ Some info I was mentioning on B12 absorption
https://forums.phoenixrising.me/thr...-2-active-forms-pure-water.90012/post-2438156
The purpose of the present study is to explore the assessment if the transdermal delivery of vitamin D is feasible. This randomized control study shows that vitamin D3 can safely be delivered through the dermal route. This route could be exploited in treating vitamin D deficiency.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3976443/
As far as topical going back to the question. Malabsorption is a factor, for example- magnesium deficiency. Magnesium is absorbed in the small intestine and in the colon; so those with intestinal or colon damage such as Crohn’s disease, irritable bowel syndrome, celiac disease, gastroenteritis, idiopathic steatorrhoea, resection of the small intestine, ileostomy patients or patients with ulcerative colitis, or autism may have magnesium deficiency due to this. Nutrient relationships can be involved such as a high calcium level also impairs magnesium absorption. In these situations, the epsom salt or a Magnesium Chloride transdermal spray therefore may be helpful by bypassing the gut absorption problems.
https://forums.phoenixrising.me/threads/is-transdermal-magnesium-effective.90305/post-2437213
What keeps coming up is the lowered zinc during inflammation/infection. If you do not make stomach acid you cant absorb zinc, and if you do not absorb zinc you do not make stomach acid as zinc, vitamin B1, and B6 are needed to produce stomach acid. Hydrochloric acid helps kill off pathogens and is required for the absorption of nutrients such as calcium, iron, and various vitamins which can all be impacted as inflammation becomes chronic so lowering the inflammation while possibly supplementing some light zinc like zinc glycinate may help - zinc amino acids complexes
https://pubmed.ncbi.nlm.nih.gov/28717982/