asthma
"I was started on it after the first acute sudden asthma attack of my life and the inhalers  weren't working... which landed me in the emergency room and had some continuing lung spasms in the weeks following this. Once on it,  for the first time in many years I was no longer waking up wheezing and with shortness of breath at night. It's been wonderful!  Fortunately, I have never had any of the side effects noted by others. I remember feeling early on that singulair  may have contributed to weight gain, but it was  very hard to tell as 
I was at that age - late 40s -  when this begins to happen naturally. If so, I certainly would have preferred to not to gain weight, but the benefits of having a "normal" life with singulair have been fantastic"