It is noteworthy that Islam is a religion which pays attention to the cleanliness, decency, and the good ap
pearance and smell of its adherents. Muslims are recommended to appear decent and clean. They also are to keep their mouth clean by using brush or siwak.
The Prophet commanded his companions to use siwak in cleaning their teeth and continued to use it while fasting. Abu Dawood and At-Tirmidhi report an authentic hadith narrated by `Aamir 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.”
Dr. Muhammad Abu Laylah, professor of the Islamic Studies & Comparative Religions at Al-Azhar Univ. states: Allah never lays hardship on us or requires of us what we cannot manage, as stated in the Qur’an. He Almighty says, (… He hath chosen you and hath not laid upon you in religion any hardship…) (Al-Hajj 22: 78)
Islam urges its followers to be clean, decent, and pleasant in their appearance and smell. In this respect, you can have a mouthwash and gargle it but be careful not to allow the water go into your stomach as best as you can. During the Prophet’s lifetime, he and his Companions used to use siwak (tooth-stick) to keep their mouth fresh and clean.
Our Prophet forbade us to eat onion, garlic, or any thing of abhorrent smell and recommended us to bathe before going to attend the congregational prayer on Friday in order not to bother anyone.
By analogy, we should keep our mouth and body clean and free of any bad or horrible smell, especially when you are working with a non-Muslim who may accuse Islam or insult Muslims because of fasting. Allah says: (Keep your duty to Allah as far as you can.) (At-Taghabun 64: 16)