Cleaning Teeth while Fasting

Amir bin Rabiah (radi Allahu anhu) said: “I saw the Messenger of Allah (sal Allahu alaihi wa sallam) cleaning his teeth with a Miswaak while he was fasting, so many times that I lost count.” [Sahih Bukhari]

Provided that no external substance is swallowed, there is no harm in cleaning the mouth and teeth by any means while fasting. Should you use a Miswaak instead of a toothbrush you will get the reward of two Sunnahs: 1) cleaning the mouth, 2) using a Miswaak.