Peace be upon you.

What is the ruling on swallowing mucus or phlegm while fasting? Does it break the fast and mean that the day has to be made up?

Please advise.

In the name of Allah, the Beneficent, the Merciful,

All praise is due to Allah and may peace and blessing be upon the Messenger of Allah, upon his household, his companions and whoever follows him. To proceed:

Swallowing mucus or phlegm does not nullify the fast, for it is not one of the things that make a person stronger, it is not enjoyable and no one wants to do it. It cannot be categorized as either of the two types of desire: neither that of the stomach nor that of the private parts. It is, of course, well-known that this is an issue of ijtihad and that the people of knowledge have two stances on it, the more preferable of the two being that this does not nullify a person's fast, and when the scholars differ, we refer back to the Qur'an and Sunnah. So, if we have any doubt in this matter—whether or not this nullifies the fast—the basic rule is that it does not nullify it. Based on that, swallowing mucus does not break the fast.

And Allah Almighty is the Most High and He knows best.