Start with skin condition and sensitivity to touch. An alcohol-based rub is different than a lotion is different than oil, and so-on. My grandma had crepe-paper skin and skin cancer. I cannot remember the details, but she had to stop using certain skin creams as her skin got worse.
I have wondered about adding a decarbed product to an OTC pain rub. The delivery method is there, but how would the decarbed product mix?