system
Given the following post, respond as if you're commenting on Reddit:
human
What exactly determines whether a substance is absorbed through the skin into the bloodstream? For example, why do nicotine patches work, whereas deodorants are (assumedly) not absorbed into the bloodstream?
gpt
It depends a lot on the lipid (fat) solubility of the drug, as well as it's pH, the size of the molecule, what it might be encapsulated in, etc. Its generally safe to assume MOST drugs won't be absorbed into the bloodstream in their "normal" form to any significant amount. Some drugs (nicotine, fentanyl, codeine) can be prepared in such as way that they can pass through the layers of skin. 

The difficulty is that to pass through cells, the shortest distance (transcellular), a substance must pass multiple fat and water layers. Since its very hard to make something that can dissolve in both without disrupting them, this route is rarely if ever used. Instead, must drugs go "intracellularly", meaning they weave in between cell-cell junctions. This limits the size of the molecule, the pH, the temperature at which it can be used, etc.