I developed asthma as an adult after moving here to Seattle. It's bad too (had ER visits). One would think it was due to allergies, but allergy testing comes up with nothing... don't react to anything at all. Am on Combivent & Advair. Doc added Singulair last year. I was on it for 6 months and noticed utterly no difference at all except having every "rare" side effect listed for the first month or so. I stopped taking it in January because why take more meds and pay more money for nothing. Again, noticed no difference. Asthma didn't get worse (or better) ;-)

So my personal experience was
Singulair = no difference.

Debbie in Seattle