The aim of this study is to assess the effects of N-acetylcysteine (NAC) on initial cessation and relapse prevention in adult cigarette smokers. Specifically, this study has the following aims: Aim 1) Examine the efficacy of NAC, compared to placebo, in helping smokers achieve three days of continuous abstinence; Aim 2) Among those who maintain initial 3-day abstinence, examine the time to relapse over the 8-week intervention between NAC and placebo groups; Aim 3) Assess 7-day point prevalence abstinence at the 8-week end-of-treatment study visit in order to obtain effect sizes and estimates of variability to power a randomized clinical trial.