Can one rinse the mouth out with mouthwash or water on a fast day? If I will be very careful not to swallow any of the liquid is there a problem? I am very uncomfortable if I am not able to rinse my mouth out in the morning. Is there any difference between yom kippur and other fasts on this question?
It is permitted to rinse one’s mouth with mouthwash, with the exception of Yom Kippur, when it is prohibited. The reason for this is that one is careful in any case not to swallow mouthwash, because it damages the body.
It is preferable not to rinse one’s mouth with water on fasts. If it is necessary (if one feels discomfort), one may be lenient, with the exception of Yom Kippur. One must be careful not to swallow the water.
Sources: See Shulchan Aruch 567:3; Darkei Moshe1; Eliyah Rabba 5; Be’er Heitev 5; Chayei Adam 132:20; Aruch Hashulchan 3; Mishnah Berurah 11; Kaf Hachaim 14; Toras Chaim 12; Dovev Meisharim 1:124 (most permit rinsing one’s mouth with water in times of need even on Tisha Be’av, but not on Yom Kippur). Concerning mouthwash, see Be’er Moshe vol. 8, permitting rinsing even on Tisha Be’av.