Dear Rabbi Simon,
Do I need a special toothbrush for Shabbat? Someone told me about this. Isn’t there a dual problem with brushing your teeth with regards to “taking the bad from the good” and “leveling” (of the toothpaste)?
General “best practice” is to refrain from using toothpaste on Shabbos, even with a special toothbrush. The issue is indeed the spreading or levelling of the toothpaste. (There are some rabbis who allow this, but prominent authorities do not endorse this leniency.) One may brush his teeth using mouthwash, however. In rinsing your toothbrush afterwards, take care not to squeeze the water trapped between the fibres of the brush.
One may also use dental floss, providing that doing so will not normally cause his gums to bleed [floss regularly and this will not be a problem] and that he has cut the floss to the desired length before Shabbat [as one may not sever the floss on Shabbat itself].
I hope this is helpful.
Rabbi Rashi Simon