Quitting smoking might not hurt either.
Easier said than done and I'm not preaching.
But speaking as a former smoker I can tell you that adding muscle and adding to overall health is much easier when you quit smoking.
I did it four years ago and I know you can too if you haven't already.
Actually I psyched myself out, told myself I was "taking a break" and it's turned into a 4 year "break" so far, I never really "quit". But I got it out of my system.
Believe me your body will react in very positive ways over time and it's easier to put on muscle mass.