It's no secret that when we fast, we often feel the need to constantly wash or clean our mouth..
But toothpaste and mouthwash have a taste, and what if we accidentally swallow it? Can we use toothpaste or mouthwash while fasting? Will it break our fast?
Shaykh Yasir Qadhi answers
The simple answer is...well yes but, meaning fasting will be broken if something goes into the throat, under the throat. So everything above the throat, everything in your mouth will not technically break the fast. However you are quite literally one millimetre away from breaking the fast. So when you put something external into your mouth, it's very difficult, it's not impossible, it's very difficult to guarantee that nothing goes down into your throat. So, I would strongly encourage you to try alternative methods, such as the miswak that our Prophet صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ would use.
The miswak , there's no danger of anything going in because it's just a brush. Or if you use a toothbrush without toothpaste, that will also not have any problem. However, if you do use toothpaste, then you have to be extra careful and conscious that nothing goes under and into your throat and that you spit everything out, and you wash your mouth very, very thoroughly so that none of the toothpaste will then go inside of your throat because that will break the fast.
So, be very, very careful and try alternative means. But if you are careful, and you make sure nothing is swallowed, then the fast will be valid.