Is it permitted to brush teeth on the 17th of Tammuz fast?

Hello,

It is preferable not to brush one’s teeth on the fast of the 17th of Tammuz. However, if refraining causes significant discomfort, one may be lenient. In such a case, care should be taken to tilt the head downward to avoid swallowing any water.

Sources:
Shulchan Aruch, Orach Chaim 567:3; Mishnah Berurah 567:11; Minchat Yitzchak, Vol. 4, Siman 109; Chazon Ovadia – Arba Taaniyot, pp. 27–28.

Blessings,
Hillel Meirs