May I brush my teeth on Shabbos?
There are myriad potential issurim (prohibitions) associated with brushing teeth on Shabbos, even with a manual toothbrush. Different poskim propose one or more of the following restrictions:
Memarai’ach (smearing): toothpaste is spread on the teeth and around the mouth.
Molid (creating something new): the texture of the toothpaste changes from cream to foam during brushing.
Chovel (causing injury): brushing may cause the gums to bleed.
Sechitah (squeezing): liquid is wrung out of the brush bristles by brushing.
Kibus (laundering/washing): the toothbrush absorbs water.
Hachanah (preparing for the weekday): the toothbrush is rinsed after use so it may be reused after Shabbos.
Refuah (using remedies): fluoride is therapeutic for the teeth.
Uvdin d’chol (weekday conduct): the device—i.e. toothbrush—is intended for everyday use.
Taking even a small number of these issues into account, a toothbrush would be considered a keli shemelachto l’issur (utensil designated for forbidden use), and therefore muktzah (may not be moved on Shabbos in the normal manner). It may only be moved on Shabbos if its place is needed (l’tzorech m’komo), or when used as a stand-in for permissible activity (l’tzorech gufo).
Today most of the pitfalls of these issurim can be avoided with the advent of products that are designated for Shabbos use: “Shabbos toothpaste”—which is a liquid application and a “Shabbos toothbrush” with soft, widespread bristles that are impermeable and firmly fixed. When conducting any routine self-care on Shabbos, we are cautioned to remember corollary issurim; in this case, the hot water tap should obviously not be used.