Vomiting and swallowing phlegm and saliva while fasting
Does vomiting and swallowing phlegm invalidate fasting? Is it permissible for a fasting person to swallow his saliva during the fasting hours of Ramadan?
If a fasting person vomits involuntarily without deliberately inducing vomiting, his fast is valid and he is not required to make it up. However, he must not deliberately swallow anything that comes out of an interior cavity and he must take care not to let this happen. There is no harm if anything slips back to an interior cavity.
The fast of a person who deliberately vomits while he remembers that he is fasting, is invalidated even if nothing slips back to his cavity and he is obliged to make up the fast of that day. This is because of the hadith of the Prophet who said, "Whoever is overcome by vomiting is not to make up his fast and whoever vomits intentionally must make it up."
Swallowing phlegm while fasting does not break the fast according to the opinion of the majority of scholars except if it reaches the mouth and is then swallowed.
It is permissible for a fasting person to swallow his saliva because jurists mentioned it among the things that do not invalidate fasting. This is due to its frequent occurrence and because it is among the things which are impossible to avoid, such as smelling fragrant scents, inhaling or swallowing road dust etc.
God Almighty knows best.
 Included by Tirmidhi (3\98), Ibn Majah (1\536) and Ahmad in his Musnad (2\498).