- Peninei Halakha
Le-khatĥila, one should not wash his mouth on the minor fasts, because there is concern that one might swallow drops of water. However, one who knows that he has bad breath may wash out his mouth, because he has no intention to drink, only to clean his mouth. Still, he should be very careful not to swallow any water. One may use toothpaste in order to clean out his mouth thoroughly and remove bad breath.
On Tisha Be-Av, which is a more stringent fast, there is an additional prohibition against washing oneself. Therefore, one should be more stringent on Tisha Be-Av and, unless it is very necessary, one should not rinse his mouth. Only one who would be greatly distressed may wash his mouth and brush his teeth, without toothpaste, even on Tisha Be-Av. On Yom Kippur, however, when one must fast by Torah law, one may not be lenient in this regard.
 While it is true that, according to sa 567:3, one may not rinse one’s mouth on a fast day, many Aĥaronim, including mb 567:11, state that it is permitted if not doing so would cause distress. This is even permitted on Tisha Be-Av, in a case of great suffering. We are more spoiled today because of a higher hygienic awareness, and refraining from rinsing one’s mouth in the morning causes most people distress. Therefore, those who suffer may wash their mouths (see below ch. 8 n. 19). Regarding toothpaste, see Berakhot 14a: “One who is fasting may take a taste, without any issue.” Most Rishonim understand that the Gemara is talking about tasting without swallowing, in order to know how the dish tastes, but one may not swallow. There is also debate as to which fast day is at issue. According to Rosh, the Gemara’s statement applies to any communal fast (except Tisha Be-Av), and sa 567:1 rules accordingly. Tosafot maintain that the Sages permitted tasting only on personal fast days, not on communal fasts. Rema writes that it is customary to be stringent and not to taste anything on a fast day. However, mb 567:6 states that one may be more lenient if he is tasting food for the sake of a se’udat mitzva. Certainly, then, one who suffers when he does not brush his teeth with toothpaste may be lenient. Perhaps even Rema would agree with this, because such a person has no intention to taste the toothpaste, only to clean out his mouth. Tisha Be-Av is more stringent, because washing in general is forbidden then, and washing one’s mouth is included in this restriction. Nonetheless, one who is greatly distressed may wash his mouth. However, he may only do so without toothpaste, because even Rosh, who rules leniently on the issue of tasting food on a fast day, forbids tasting on Tisha Be-Av. We rule even more stringently on Yom Kippur, because it is prohibited to eat and drink on Yom Kippur – even a ĥatzi shi’ur (an amount below the threshold of punishment) – by Torah law.