Generally, if any kind of medicine enters the body while fasting, the fast is deemed invalid. An inhaler contains medication, such as liquid Ventolin, which enters the body when the inhaler is pumped, and thus, according to the ruling stated above, using an inhaler deems the fast invalid.
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دلائل:
الدر المختار: (395/2، ط: سعید)
ومفادہ انہ لو ادخل حلقہ الدخان افطرای دخان کان ولو عوداً او عنبراً لو ذاکراً لا مکان التحرز عنہ۔
Allah the Almighty knows best.
Issued by Darul ifta Al-Ikhlas, Karachi.