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Background and Objective: Nicotine metabolic ratio (NMR) has been associated with nicotine metabolism and smoking characteristics. However, there are few studies on the potential association between NMR and smoking cessation efficacy in smokers with chronic obstructive pulmonary disease (COPD) in China or elsewhere., Methods: This study was a stratified block randomized controlled trial for smoking cessation in Chinese smokers with COPD. NMR was used as a stratification factor; slow metabolizers were defined as those with NMR <0.31, and normal metabolizers as those with NMR ≥0.31. Participants were randomly assigned to the varenicline or bupropion group. Follow-up visits were conducted at 1, 2, 4, 6, 9, 12 and 24 weeks., Results: Two hundred twenty-four participants were recruited and analysed from February 2019 to June 2022. In normal metabolizers, the 9-12 weeks continuous abstinence rate of varenicline (43.1%) was higher than in bupropion (23.5%) (OR = 2.47, 95% CI 1.05-5.78, p = 0.038). There was no significant difference in abstinence rates between treatment groups in slow metabolizers (54.1% vs. 45.9%, OR = 1.39, 95% CI 0.68-2.83, p = 0.366). For slow metabolizers, the total score of side effects in the varenicline group was significantly higher than the bupropion group (p = 0.048), while there was no significant difference in side effects between groups for normal metabolizers (p = 0.360)., Conclusion: Varenicline showed better efficacy than bupropion in normal metabolizers, and bupropion showed equivalent efficacy in slow metabolizers with less side effects. According to our study, NMR provides a better justification for both scientific research and tailoring optimal pharmacotherapy for smoking cessation among smokers in COPD., (© 2024 Asian Pacific Society of Respirology.)