Is it permissible to rinse one’s mouth out with water on a fast day?
Although it is forbidden to rinse one’s mouth on Yom Kippur and Tisha B’Ab, one may rinse his mouth with water on the other fast days, as long as he uses less than a Rebi’it (3.1-3.2 ounces) of water, and he ensures to expel all of it and not swallow any water. Hacham Ovadia Yosef (Hazon Ovadia – Ta’anit, p. 28) writes that on a private fast day which an individual accepts upon himself, he may rinse his mouth with even more than a Rebi’it, as long as he ensures not to swallow any of it.
When it comes to chewing gum on a fast day, Hacham Ovadia rules that if the gum has a sweet flavor (like most chewing gum does), then it may not be chewed on a fast day. Flavorless gum may be chewed, even it has the effect of increasing the amount of saliva in the mouth, since saliva is considered part of the body and it may be swallowed on a fast day.
A person who experiences a headache or other kind of pain on a fast day may take a pill, such as an Advil or Tylenol, as long as it is flavorless. Hacham Ovadia (ibid. page 30; listen to audio recording for precise citation) rules that one should swallow the pill without water, but if he is unable to, then he may drink a bit of water to help him swallow the pill, and he is still considered to have observed the fast.
Summary: It is permissible to rinse one’s mouth with a bit of water on fast days, with the exception of Yom Kippur and Tisha B’Ab. Gum may not be chewed on fast days unless it is flavorless. One may swallow a pill if needs to on a fast day, and if necessary he can take a sip of water with the pill.
