Praise be to Allaah.
Shaykh Ibn Baaz (may Allaah have mercy on him) was asked
about using toothpaste whilst fasting. He said: there is nothing wrong
with that so long as one is careful to avoid swallowing any of it, just
as it is prescribed in Islam for the fasting person to use the siwaak
(tooth-stick)
Fataawa al-Shaykh Ibn Baaz,
4/247
Shaykh Muhammad al-Saalih ibn ‘Uthaymeen said:
… This leads us to the question: Is it permissible to
use a toothbrush and toothpaste whilst fasting or not?
The answer is that it is permitted, but it is preferable
not to use them because the toothpaste is too strong and may go down
into the throat. Instead of doing that during the day, one should do
it at night.
Al-Sharh al-Mumti’ by Ibn ‘Uthaymeen, 6/407, 408