Can one use deodorant on Yom Kippur? What if the reason is to remove sweat? Is the gel, spray, or stick all assur?
It is best to avoid deodorant on Yom Kippur.
Under circumstances of major discomfort, it is permitted to use spray deodorant, but not gel or stick deodorant, which can be considered sichah (anointing). Sichah on Yom Kippur is prohibited even for the purpose of removing bad odor (614:1).
The spray is not sichah, and therefore permitted, but poskim (cited from Rav Elyashiv; also piskei halachos of R. S. Develetzki) nonetheless write that as a ‘cosmetic’ one should not use it.