I am going to do some copying and pasting of what i wrote on PR in responses to posts.
There are MANY ways to go about this. I have found through research that approximately 70% is being absorbed into blood stream...varies on what it is, density, evaporation etc. This is compared to about 85% internal absorption, although highly unlikely the shape my gut is in has been absorbing that level. There are patch companies that make certain vitamins, meds etc through transdermal methods. Due to my mast cell being so severe i had to go about things very differently and add only certain vitamins, one drop at a time.
Using your intuition will go a long way. There is not much guidance in this area yet. I can only tell you how i went about it. I seemed to be showing serious signs of berberi, scurvy and severe vit. d deficiency in 2018/2019 confirmed by my lyme practioner. Yet i was having serious reactions to anything internal and often times through IV. So, i began this journey out of necessity to save my life. I have found inner thighs works well as the vitamins or meds have no place to go really before they are eventually absorbed. I also use abdomen, buttox, inner arms, over thyroid (only certain ones like zinc), feet and the less hair= more absorption. Honestly if i had someone to regularly put this on my spine, thats an ideal place as well. Depending on how severe your situation is and what frequency you need applications, you can wipe areas about 2 hours later with warm washcloths to keep skin clean. I am having issues with B1 causing mast cell reactions and skin sores...other than that its gone very well. I still need to get this resolved though. Always something!
You can use a pill crusher or just back of spoon and a plate etc. They sell glass dropper bottles on amazon and you can add a few drops of water to the pill and let it dissolve then add to skin, etc. You can go as slow or fast as you want to be observant of reactions etc. Most vitamins are available in liquid form...singles and many others.
Do not forget you can absorb MUCH through your feet too. In 2017 when i was stronger i was doing rosehip foot soaks with ancient minerals magnesium flakes and i was absolutely upping my c and mag levels. I unfortunately don't have access to frequent foot soaks anymore. If its an option for you, look into it. Its one of the best ways to get magnesium in body with very little intestinal effects. Ancient minerals also makes a fantastic magnesium lotion as well. Magnesium is needed by so very many of us, especially with this illness. I have tolerated it transdermally EONS better than all my internal attempts that produced either bad GI or neuro/agitation/inflammation issues.
I also want to add a good way to test these items on you is one drop on the wrist and put your wrists together and gently rub in clockwise motion (this also soothes the HPA axis and nervous system and tells your body all is well). In general, i am quite sensitive and can sometimes feel just by holding the bottle if i gel with something or not. I will feel strange sensations up and down my arm if i do not gel or hives/redness etc if i apply to skin. Depending on how severe you are, you can wait a few days and see how you do with that one drop then proceed from there or you can add more if you feel its safe to proceed. Again, intuition is key. Our bodies are really so much more intelligent than we give them credit for. You can also use this testing method when introducing a food you are unsure of etc.
I do hope this is helpful. Please write below if you would like me to further explore one of these areas etc.
Does anyone have any specific questions or stories of their own they would like to share in comments?