Using Toothpaste while Fasting

Q: Is it permissible to brush my teeth with toothpaste while fasting?

A: In principle, toothpaste is closest in ruling to chewing gum that partially dissolves in the mouth, which is impermissible while fasting and invalidates the fast if its flavor reaches the throat.

Toothpaste, by design, is intentionally placed in the mouth, spread across the teeth and tongue, mixed with saliva, and worked for an extended period. Its ingredients are specifically formulated to coat the mouth and leave a lingering taste. Even after rinsing, flavor commonly remains.

Because of this, the likelihood that its taste reaches the throat is very high, even if a person does not intend to swallow anything.

The jurists apply the principle:

يُقِيمُونَ الْمَظِنَّةَ مَقَامَ الْمَئِنَّةِ

They treat likelihood as certainty.

When an act is structured in a way that internal entry is highly probable, the law treats that probability as if it has already occurred.

For that reason, toothpaste is not permitted while fasting.

Source: Sharh al-Muntaha & Kashshaf al-Qina