Brushing Teeth on the Fast of the 10th of Tevet
Question
Answer
Hello,
Brushing with water or toothpaste is prohibited. It is also forbidden to rinse the mouth after brushing, so it is advised to refrain from brushing.
Someone who is particularly sensitive or experiences significant discomfort if they do not brush may brush and rinse their mouth, but must be careful to spit everything out and also tilt their head downward to avoid swallowing even a small amount of water.
Source
Shulchan Aruch, Orach Chaim, Siman 567, סעיף 3
One who usually rinses their mouth in the morning should not do so on a fast day.
Mishna Berura, Siman 567, סעיף קטן 11
(11) To do so - In cases of significant discomfort, rinsing the mouth with water is permitted on any fast day, but one must be very careful to tilt the head and mouth downward to prevent water from reaching the throat.