© 2018 American Medical Association. All rights reserved. IMPORTANCE: The optimal temporal approach for reducing nicotine to minimally or nonaddictive levels in all cigarettes sold in the United States has not been determined. OBJECTIVES: To determine the effects of immediate vs gradual reduction in nicotine content to very low levels and as compared with usual nicotine level cigarettes on biomarkers of toxicant exposure. DESIGN, SETTING, AND PARTICIPANTS: A double-blind, randomized, parallel-design study with 2 weeks of baseline smoking and 20 weeks of intervention was conducted at 10 US sites. A volunteer sample of daily smokers with no intention to quit within 30 days was recruited between July 2014 and September 2016, with the last follow-up completed in March 2017. INTERVENTIONS: (1) Immediate reduction to 0.4 mg of nicotine per gram of tobacco cigarettes; (2) gradual reduction from 15.5 mg to 0.4 mg of nicotine per gram of tobacco cigarettes with 5 monthly dose changes; or (3) maintenance on 15.5 mg of nicotine per gram of tobacco cigarettes. MAIN OUTCOMES AND MEASURES: Between-group differences in 3 co-primary biomarkers of smoke toxicant exposure: breath carbon monoxide (CO), urine 3-hydroxypropylmercapturic acid (3-HPMA, metabolite of acrolein), and urine phenanthrene tetraol (PheT, indicator of polycyclic aromatic hydrocarbons) calculated as area under the concentration-time curve over the 20 weeks of intervention. RESULTS: Among 1250 randomized participants (mean age, 45 years; 549 women [44%]; 958 [77%] completed the trial), significantly lower levels of exposure were observed in the immediate vs gradual reduction group for CO (mean difference, -4.06 parts per million [ppm] [95% CI, -4.89 to -3.23]; P <.0055), 3-HPMA (ratio of geometric means, 0.83 [95% CI, 0.77 to 0.88]; P <.0055), and PheT (ratio of geometric means, 0.88 [95% CI, 0.83 to 0.93]; P <.0055). Significantly lower levels of exposure were observed in the immediate reduction vs control group for CO (mean difference, -3.38 [95% CI, -4.40 to -2.36]; P <.0055), 3-HPMA (ratio of geometric means, 0.81 [95% CI, 0.75 to 0.88]; P <.0055), and PheT (ratio of geometric means, 0.86 [95% CI, 0.81 to 0.92]; P <.0055). No significant differences were observed between the gradual reduction vs control groups for CO (mean difference, 0.68 [95% CI, -0.31 to 1.67]; P =.18), 3-HPMA (ratio of geometric means, 0.98 [95% CI, 0.91 to 1.06]; P =.64), and PheT (ratio of geometric means, 0.98 [95% CI, 0.92 to 1.04]; P =.52). CONCLUSIONS AND RELEVANCE: Among smokers, immediate reduction of nicotine in cigarettes led to significantly greater decreases in biomarkers of smoke exposure across time compared with gradual reduction or a control group, with no significant differences between gradual reduction and control.