Shaykh Ibn Uthaymeen explained that the foundational principle is that a person's fast is valid and sound unless they commit one of the acts that are known to invalidate it. The invalidators of the fast (mufattirat) are established by the Qur'an and Sunnah.

The primary invalidators include:

1. Deliberate eating and drinking.
2. Sexual intercourse.
3. Anything that takes the place of eating and drinking, such as nutritional injections or IV drips.
4. Menstruation or postpartum bleeding.
5. Deliberately causing oneself to vomit.

Regarding the specific issues raised:

• Swallowing Saliva: This does not break the fast. It is a natural and unavoidable bodily function. If a person were to try to avoid swallowing it, this would cause them undue hardship, and Islam is a religion of ease. Therefore, swallowing one's own saliva, phlegm, or mucus that has not come into the mouth is permissible and does not affect the fast.

• Brushing Teeth: Using a siwak is Sunnah at all times, including for the one who is fasting. Using a toothbrush and toothpaste is also permissible, provided that the person is careful to avoid swallowing any of the toothpaste. If any of it is swallowed accidentally, the fast is still valid, but deliberate swallowing would break it. It is better to be cautious.

• Asthma Inhaler: Shaykh Ibn 'Uthaymeen issued a fatwa that using an asthma inhaler does not break the fast. He reasoned that the content of the inhaler is a compressed gas that goes primarily to the lungs to open the airways; it is not food or drink. It is a medical necessity, and since it is not nutritive and does not enter the stomach as food does, it does not invalidate the fast. The fast remains valid.

Other things that do not break the fast include non-nutritional injections (e.g., penicillin, insulin), having blood taken for a test, and using eye or ear drops, as they are not considered food or drink.