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Urine 2-naphthol concentration during progressive yield reduction of cigarettes (weeks 1–6) and after return to usual cigarettes or quitting (weeks 7–10). Yields in the boxes refer to FTC machine-determined nicotine yield. Solid line includes all subjects; dashed line excluded two quitters at week 10. Points, mean for 20 subjects; bars, SE. * P < 0.05 compared to viral brand.
