Praise be to Allah
The scholars are unanimously agreed that if mucus and
phlegm pass the throat of the fasting person without him being able to
expel them, that does not invalidate the fast, because that is beyond the
fasting person’s control.
Shaykh Zakariyya al-Ansaari ash-Shaafa‘i (may Allah have
mercy on him) said:
If the mucus comes from the mouth or nose and goes down into
his throat and he is unable to expel it, that does not break the fast,
because he is excused.
End quote from Asna’l-Mataalib (1/415)
But if the fasting person swallows it after being able to
bring it out of the throat (to the mouth), some of the scholars, such as
Imam ash-Shaafa‘i, are of the view that the fast is invalidated thereby. But
the view of Abu Haneefah and Maalik, and one view narrated from Ahmad, is
that it does not invalidate the fast. This view is the one that is regarded
as more likely to be correct by Shaykh Ibn ‘Uthaymeen (may Allah have mercy
on him).
See: al-Majmoo‘ah al-Fiqhiyyah (36/259-261)
Ibn Nujaym al-Hanafi (may Allah have mercy on him) said:
If mucus enters the fasting person’s nose from his head, then
he sniffs it up and it enters his throat, as a result of a deliberate action
on his part, that does not affect his fast, because it is like his saliva.
End quote from al-Bahr ar-Raa’iq Sharh Kanz ad-Daqaa’iq
(2/294)
An-Nafraawi al-Maaliki (may Allah have mercy on him) said:
Phlegm that comes up from the chest to the back of the tongue
and is swallowed does not require the fast to be made up, even if it is
possible to expel it. The same applies to mucus, even if it reaches the back
of the tongue and is deliberately swallowed; it is not required to make up
the fast in any of these cases.
End quote from al-Fawaakih ad-Dawaani (1/309)
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
If the mucus does not reach the mouth, in the sense that the
person feels it coming down from inside his head, and it goes to his throat,
that does not break the fast. That is because it has not reached the outside
of the body, and the mouth comes under the same ruling as the outside. But
if it reaches the mouth then he swallows it after that, that does break the
fast. But if it does not reach the mouth, then it still comes under the
ruling of what is inside, so it does not break the fast.
There is another view concerning this issue, which is that it
does not break the fast even if it reaches the mouth and he swallows it.
This view is more likely to be correct, because it did not come out of the
mouth, so swallowing it is not regarded as being like eating or drinking.
End quote from ash-Sharh al-Mumti‘ (6/424)
To sum up: your fast is not invalidated because of the
effects of sinusitis, such as mucus, blood and the like. But if you are able
to expel it and spit it out, that is preferable, so as to be on the safe
side with regard to your fast.
We ask Allah to grant you healing and well-being.
And Allah knows best.