If no visible (to the naked eye) droplets come out then it is fine and does not break your fast, such as a vicks vapoinhaler. An asthma inhaler emits a visible spray so it does break your fast, but you should use it if you need it and make up that day.
What the Scholars Say
Contemporary scholars have disagreed whether an asthma inhaler will break your fast. One group says it will because it falls into the category of any visible substance being intentionally swallowed. The other group says it does not because it is intended to go to the respiratory system and not the stomach, so the tiny amount that reaches the stomach is unintentional and excused, just like when you rinse your mouth and some water unintentionally reaches your stomach.
I think both views are quite strong but slightly lean towards the opinion that it breaks the fast since it is still a substance that is intentionally ingested through the mouth.
Shaykh Mustafa Umar