SHAFAQNA – The Grand Ayatollah Makarem Shirazi answered questions about brushing teeth whilst fasting.
Question: Is it allowed to brush teeth whilst fasting? What is the ruling if the wet tooth brush touching saliva used again to clean teeth?
The Grand Ayatollah Makarem: There is no problem brushing the teeth whilst fasting. But if it gets wet and the brush is taken out and retued to the mouth, and the wetness does not disappear in saliva, is not allowed, and if (the person) brushes with tooth paste, must wash the mouth completely with water.
Source: PERSIAN SHAFAQNA