Reference: Fataawa Ramadhaan – Volume 2, Page 496, Fatwa No.446
al-Fataawa libni ‘Uthaymeen – Kitaab ad-Da’wah – Volume 1, Page 168
Question: What is the ruling regarding using toothpaste whilst fasting during the day in Ramadhaan?
Response: There is no harm for the fasting person to use toothpaste so long as it (toothpaste) does not [pass his throat and thereby] reach his stomach. However, it is better not to use it because it has a strong penetrating [taste] and therefore could quite possibly reach the stomach while the person is unaware of it. Because of this, the Prophet (sal-Allaahu ‘alayhi wa sallam) said to al-Qayt ibn Saburah:
«[Whilst making wudhoo] exert yourself in sniffing water up the nose except when you are fasting»
So, it is better for the fasting person not to use toothpaste; And the matter [of cleaning the teeth] can be accommodated; if he were to delay it until he broke his fast [at sunset] then he has protected himself from that which one fears may spoil his fast.