Can I Whiten My Teeth while Fasting?
Question

What is the ruling on using a teeth whitening kit during the day in Ramadan, noting that it contains a substance which is put on the teeth to whiten them, and that process lasts for several hours. Is it permissible to use it?
Praise be to Allah.Can I whiten my teeth while fasting?
Teeth whitening kits include moulds in the shape of the teeth, inside which there is a whitening substance. These moulds are worn over the teeth and left there for a certain length of time.
The impact that these kits have on the soundness of the fast depends on whether the substance leaks from these moulds into the interior of the mouth.

The first scenario is when they are tightly placed over the teeth and gums, so that nothing comes out into the mouth, or if it comes out the fasting person is able to spit it out. In this case, these moulds do not affect the validity of the fast, because there is nothing that causes the fast to be broken.
The second scenario is when the substance leaks out and the fasting person swallows it. In this case it is not permissible for the fasting person to use them.

Can you brush your teeth while fasting?
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said: With regard to using a toothbrush and toothpaste when fasting , one of two scenarios must apply:

In the first scenario, the toothpaste is very strong and could reach the stomach, and the user is unable to control it. This is prohibited, and it is not permissible to use it, because it leads to breaking of the fast, and whatever leads to something that is haram is also haram. According to the hadith of Laqit ibn Saburah, the Prophet (blessings and peace of Allah be upon him) said to him: “Rinse your nose thoroughly, unless you are fasting.” The Prophet (blessings and peace of Allah be upon him) made an exception to rinsing the nose thoroughly if one is fasting, because if a person rinses his nose thoroughly when fasting, the water may leak into his throat, thus invalidating his fast. Hence we say: if the toothpaste is strong and will reach the stomach, then it is not permissible to use it in this situation, or at the very least we say that it is makruh (disliked).
In the second scenario, if it is not that strong, and it is possible to avoid letting it reach the stomach, then there is nothing wrong with using it, because the inside of the mouth comes under the same ruling as the outside, hence a person may rinse his mouth with water and that does not affect [the fast]. If what is inside the mouth came under the same ruling as the inside of the body, then the fasting person would not be allowed to rinse his mouth." (Majmu‘ Fatawa ash-Shaykh Ibn ‘Uthaymin 16/351)

For more, please see the answers: 292125 and 363474 .
And Allah knows best.