The inhalers used by individuals suffering from asthma and other pulmonary conditions deliver an extremely small quantity—approximately 1/20 milliliter—into the mouth with each actuation. A substantial portion of this amount is absorbed by the lining of the oral cavity and the respiratory tract and thus effectively dissipates. There is no definitive evidence establishing that any residual amount remains and reaches the stomach via saliva.
When compared to the water that may remain in the mouth during ablution (wuḍūʾ), the quantity involved in the use of such inhalers is significantly smaller. Yet it is established through a Prophetic report (Dārimī, Ṣawm, 21) and through the consensus (ijmāʿ) of Islamic jurists that if a trace amount of water remaining in the mouth after ablution reaches the stomach, the fast is not thereby invalidated.
In light of these considerations, provided that their general state of health permits fasting and they suffer from no additional condition that would preclude it, the use of oxygen-based inhaler medication sprayed into the mouth for the purpose of facilitating breathing does not invalidate the fast.