Reference: Fataawa Ramadhaan v2 p540 no.516
al-Fataawa libni ‘Uthaymeen – Kitaab ad-Da’wah v1 p170
Question: Does the use of gargling medication invalidate the fast?
Response: So long as he does not swallow any of the gargling medication, his fast is not invalidated. However, he should not use it unless it is necessary.
