Ducky's right--you should contact your rheumy immediately. You're on a fairly high dose of mtx, and it sounds like you've been taking it a while. I developed a cough within 3 weeks of starting mtx--rheumy took it seriously and I had to have a full evaluation by a pulmonologist. As it turns out, it was not the mtx, but some sort of weird viral infection (perhaps viral pneumonia) that has scarred my lungs, although it has not altered my lung functioning. The ramifications of letting the cough go are too risky--whether the cough is due to mtx or to an infection. Any kind of infection (viral or bacterial) can get out of control when you're taking immunosuppressant drugs. I'm not trying to be alarmist, but what can it hurt to simply call your doctor?
I used to smoke as well--I quit 5 years ago. I was also scared of what I might have heard when I went to the pulmonologist. But, ultimately, I had to go anyway. You would not necessarily be sick (or feel sick) if mtx were affecting your lungs. The lung infection I had didn't make me feel sick at all--I had no fever, no fatigue, nothing--just a dry cough that got worse at night. And yet, it still scarred my lungs.
I would still recommend that you get it checked out anyway. Good information to know. Plus, when I got my evaluation done, my rheumy said it was excellent that now I had a very extensive work up on my lungs--more than most people get in a lifetime. So now, if anything were to happen to my lungs, I have a great baseline for comparison purposes.
Make the call anyway--worry about quitting smoking some other time. What you have going on could be absolutely nothing.
I was on mtx for a year - I had a horrible tickle/pain in my chest and throat. I had a persistant cough. I couldn't walk by someone smoking w/o hacking for the next hour and then felt rotten for the rest of the day. I felt like my lung capacity shrunk by 1/2 - I suddenly couldnt walk the stairs w/o huffing and puffing. I always felt like I was coming down with something constantly (you know that "I don't feel right today" feeling, only it was every day.) They checked me with xrays, breathing test, etc. and claimed I was "not allergic to methotrexate" nor were my problems "methotrexate releated" and said I had plurisy (sp?)
It was a bunch of crap. I had never had those problems before I started it and once I stopped the med,the problems. went away. Get it checked out.