[The ruling on a fasting person’s use of an asthma inhaler]

Answer—and Allah is the Granter of success: What appears is that using the inhaler for asthma does not invalidate the fast when used in a case of necessity and need. This is indicated by several points:
1- It is a subtle treatment, of the genus of smoke and vapor—and even subtler than dust; concessions have come regarding these because avoiding them causes hardship and difficulty, and the hardship of need here is greater than the hardship of mere exposure to dust and smoke.
2- The asthma treatment (the inhaler) does not run along the path of food and drink; rather, it runs along the path of air—the trachea and the lungs. The prohibited things that invalidate the fast are eating, drinking, and intercourse.
What runs along the path of air is not termed “eating” or “drinking.”
3- Allah the Exalted said: “Allah intends for you ease and does not intend for you hardship,” [al-Baqarah:185] and, “And He has not placed upon you in the religion any hardship.” [al-Ḥajj:78].
If it be said: Among the ease in our upright religion is that the sick person may break his fast and then make it up, as He said, Exalted is He: “And whoever is ill or on a journey—then [fast] a number of other days. Allah intends for you ease and does not intend for you hardship.” [al-Baqarah:185].
We say: Asthma is a chronic illness from which a sufferer seldom recovers fully; he often cannot make up the fasts because the illness attends him continually. The better course is that the sufferer fast and use the inhaler. Allah the Exalted said: “So fear Allah as much as you are able,” [al-Taghābun:16], and, “Allah does not burden a soul beyond its capacity.” [al-Baqarah:286].
If it be said: Feeding the needy (fidya) stands in place of fasting for one from whom recovery is not expected.
We say: Fasting is preferable for the asthma sufferer, because he is able and capable of abstaining from food, drink, and intercourse—which is the very reality of fasting—and by fasting he attains the benefits and wisdoms for which Allah legislated fasting.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1