As far as I´m concerned there could be an issue regarding making oneself bleed, by using a toothbrush.
Are there any other problems that may arise by brushing teeth with a toothbrush in Shabbes?
What do the poskim say about this?
Mi Yodeya is a question and answer site for those who base their lives on Jewish law and tradition and anyone interested in learning more. It only takes a minute to sign up.Sign up to join this community
Rav Ovadiah Yosef held that the only halchic problem was uvda dechol, and stated that one must use a toothbrush set aside specifically for shabbos to deal with that problem.
Others held there was an issue with Dosh/Sechitah/Melaben (squeezing bristles), Memareach (smoothing the teeth or the toothpaste), or Chovel (causing the bleeding).
Regarding Dosh, the gemara says "ayn sechitah base'ar," and there is a debate whether non-absorbent nylon bristles are therefore exempt, and whether the water that gets "squeezed out" in this manner actually fits the parameters of the issur of Sechitah.
Memareach has a similar debate as to whether we apply it to something that is holech le'ibud. It's not really a problem with food, for instance - no one forbids spooning mashed potatoes.
For all of these, there are questions of davar she'ayno mitkaven and pesik reisha delo nicha lei (especially with Chovel).
So some said it was completely assur, some said it was basically completely muttar, and some said you needed to circumvent these issues with special shabbos toothbrushes and/or toothpaste (though this opinion isn't quite so popular in ChL"A as it is in Israel).
There are several of the 39 melachas that are at risk of being committed if you brush your teeth in a normal way. A summary are:
Smoothing the teeth. This is disputed and some say is not an issue.
Kneading (I think) if you take solid toothpaste and liquify it. So one should liquify the toothpaste before Shabbat or buy an already liquid one.
Extracting water from the toothbrush if its bristles are close enough together to hold water and if you wet it before brushing.
Causing bleeding. A toleda of Shechita. If your gums normally bleed when you brush it becomes a P'sik Reisha. However if you usually do not bleed you do not need to worry about it as it is not inevitable nor a desired consequence.
There are techniques, and special toothbrushes and toothpastes available. I usually liquify my toothpaste before Shabbat, then put it on my teeth with my hands and brush with a dry toothbrush and don't wash the brush afterwards. My gums do not normally bleed during brushing. (If yours do you need to visit your oral hygienist, by the way).
However my answer is not intended to tell you how to brush teeth but to answer the actual question of the possible issurim.
There is also the issue of cleaning the toothbrush on Shabbat morning if you don't need it again until after Shabbat. R Avraham Yossef (following his father R Ovadia Yossef) says you have to leave the toothbrush in a glass of water to avoid preparing for yom chol during shabat.
With regard to using toothpaste, I believe Rav Moshe Feinstein ruled that there was a concern of memareyach (smoothing) though i think I heard that Rav Yisrael Belsky said he was misled regarding the physical reality and that this should not in fact be a concern since there is no significant residue that remains on the teeth. Additional concerns have been raised regarding cleaning the brush (libun) or extracting liquid from the brush (dosh) though these concerns also are at best controversial. I believe Rav Yosef Dov Soloveitchik is quoted as being lenient regarding brushing one's teeth with a toothbrush with or without toothpaste. See also: http://halachipedia.com/index.php?title=Brushing_Teeth_on_Shabbat