Or break the Ramadan fast. Sharia Alert for asthma sufferers: "Ask the scholar: Using asthma sprays during fasting in Ramadan," by Mohammed Al Sadafy from Emirates 24/7, July 21 (thanks to The Religion of Peace):
An asthma patient has asked whether the Shariah permits use of medical sprays during fasting in Ramadan.
According to the fatwa section of the Department of Islamic Affairs and Charitable Activities, if the asthma patient uses a medical spray during fasting, it may go from the nasal cavity to the throat which will make fasting invalid and make re-fasting for this day necessary.
If this happens, he has to continue to refrain from eating, drinking and smoking for the rest of the day to show respect for the holy month and people who are fasting.
If an asthma patient is cured of the ailment, he should re-fast the missed days. If he has not been completely cured, he should re-fast on days when he is not suffering from asthma.
If he is a life-long asthma patient, it is no sin if he could not re-fast the missing days because his mistake was not deliberate. But it will be a sin if he did not re-fast despite being able to later.