I smoked cigarettes for 25 years and quit 5/1/01. I've noticed changes in my physiology, in particular an inclination to a form of mental and physical lethargy that I must consciously combat, because my system was keyed to the additional stimulation for all those years. I really need that (stiff) coffee now! It's pretty well known that the cig companies put various stuff in the cigarettes to keep their revenue sources addicted not just to the tobacco, but to their brand.

Not to be a hypocrite, I must say that I still puff on a pipe about once a day, usually in the evening.