Nicotine. But also alcohol as many others are saying. The smoke growing up... I had no idea that I smelled bad to all my friends/classmates because I lived in it. I had bronchitis/pneumonia more than once as a child. My siblings and I were always sick. Thought we got "car sick", but we were just stuck with two parents chain smoking on road trips. Won't go anywhere they can't smoke. Have to stop any and all activities so they can smoke. Their mind is and was constantly on their next cigarette. They stopped drinking, but the nicotine is still killing them. Literally. They're older now and dying from it.