Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear questioner, we would like to thank you for the great confidence you place in us. We implore Allah Almighty to accept our fasts and righteous deeds and grant us forgiveness by the end of Ramadan.
It goes without saying that Islam is a religion which pays attention to the cleanliness, decency, and the good appearance and smell of its adherents. Muslims are recommended to appear decent and clean. They also are to keep their mouth clean by using a brush or siwak (tooth stick, also known as miswak).
The Prophet commanded his companions to use siwak in cleaning their teeth and continued to use it while fasting. Abu Dawud and At-Tirmidhi report an authentic hadith narrated by `Amir ibn Rabi`ah. He said: "I have seen the Messenger of Allah (peace and blessings be upon him) using a tooth-stick (siwak) while he was fasting more often than I could count."
In his response to the question in point, Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
It is absolutely fine to brush one’s teeth in the morning with toothpaste while fasting so long as one takes care not to swallow the paste. It is reported in the traditions that the Prophet (peace and blessings be upon him) was seen brushing his teeth quite a number of times during the day while he was fasting.
Toothpaste is not intended for consumption. Fast is only broken when it goes directly into our system. To say that one cannot use toothpaste is nothing but rigidity. The Prophet (peace be upon him) warned us against rigidity: “Woe to those who practice rigidity; woe to those who practice rigidity; woe to those who practice rigidity.”
By avoiding brushing with toothpaste, we end up offending the public by the foul odors of the mouth. Remember it is far more important for us to avoid such objectionable things than being rigid on such frivolous interpretations which have no valid ground in Islam.
Excerpted, with slight modifications, from: www.islam.ca
The eminent Muslim scholar, Sheikh Yusuf Al-Qaradawi, adds:
Using a brush while one is fasting is not blameworthy, provided that no toothpaste or similar substances reach the stomach. If anything reaches the stomach, it breaks one's fast according to the majority of Muslim jurists.
If one is cautious and brushes his teeth and out of forgetfulness some traces reach his stomach, he is not sinful, as Allah forgives the mistakes of Muslims that are made unintentionally. Allah, Exalted be He, says: (And there is no sin for you in the mistakes that ye make unintentionally.) (Al-Ahzab 5) And the Prophet (peace and blessings be upon him) is reported to have said: "My nation shall not be held accountable for the mistakes they do unintentionally, out of forgetfulness, and under duress."
You can also read:
Ramadan: Hope Springs Eternal!
Fasting: Meaning & Rules
Does Swallowing Our Own Saliva Invalidate the Fasting?
Intentional Vomiting & Fasting
What Invalidates the Fast
Allah Almighty knows best.