Hawzah News Agency – Q: If a person who is fasting bleeds in the mouth, does it invalidate his fast?

A: Bleeding in the mouth does not void a fast. However, it is obligatory to prevent blood from reaching the throat.

Q: My gums often bleed and the blood gets mixed with saliva. At times I am not sure whether the saliva that enters my throat is mixed with blood or not. Please tell me what I am supposed to do to overcome this problem.

A: If the blood from your gums dissolves in the saliva, then the saliva is pure and can be swallowed. If you are not sure whether the saliva is mixed with blood or not, it can still be swallowed without affecting the fast.

Source: https://www.leader.ir/en/book/32/Practical-Laws-of-Islam