Reference: Fataawa Ramadhaan – Volume 2, Page 540, Fatwa No.516
al-Fataawa libni ‘Uthaymeen – Kitaab ad-Da’wah – Volume 1, Page 170
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.