Is it allowed to stick on a patch with caffeine or nicotine on Yom Kippur or is that like taking medicine.
It is controversial. Some poskim say that it is permitted to use the patch (or a suppository) because it is considered maachal breiim, since many people use them when that are not sick at all, however most other poskim say that it is still considered similar to refuah and not permitted. See sources. If you are not feeling well as a result of not taking the caffeine and you would have to stop what you are doing to get into bed, then you would be allowed to use them. In addition, if you will get a bad headache if you don’t use them, you don’t have to wait until the headache sets in to help yourself, you may take it beforehand.
(As a side point if you will have to use them on Yom Kippur, it would be preferable though, to untape and retape the stickers before yom kippur to avoid any issue of koreya with the stickers.)
Shulchan Shlomo 321-23, L’torah V’lhorah 9 pg. 13, (R’ Moshe Feinstein zt”l) that a person that is addicted to caffeine may not take a caffeine suppository unless he he will become ill enough to warrant staying in bed, or if his whole body will become weak from it as a result. Refuas Yisroel pg. 36, Minchas Yitzchok 3-21, Oz Nidberu 14-10, Tzitz Eliezer 8-15 chap. 15-14 rule stringently. On the other hand See Be’er Moshe 1-33(19), Chut Shani 4-89 (2), Kol Hatorah 61- pg. 62 from R’ Falk shlit”a, R’ Y. Berkowitz shlita rule leniently on this.