Cigarette smoking is a complex multi-step behavior involving initiation, regular smoking, nicotine dependence, cessation, and relapse. Some regular smokers maintain low-level smoking without developing symptoms of dependence,5 while others become heavily dependent smokers and experience the most difficulty with cessation and the highest risk of relapse.6, 7 Nicotine dependence has high heritability (estimates up to 75%),8, 9 and besides reducing the likelihood of quitting smoking, it is predictive of withdrawal severity,10 response to treatment,11 and smoking-related health outcomes.12, 13
