Dose–response relationship between cotinine levels and female reproductive lifespan

Abstract Background Tobacco exposure is a major public health concern and has been implicated in accelerated female reproductive aging. However, most evidence relies on self-reported smoking history, which may introduce bias. Cotinine, a reliable biomarker of nicotine exposure, provides an objective measure to clarify the association between tobacco exposure and reproductive lifespan (RLS). Methods We analyzed 11,944 women from two nationally representative cohorts: NHANES ( n = 6,081, U.S., 1999–2018) and KNHANES ( n = 5,863, Korea, 2014–2020). Serum cotinine (NHANES) and urinary cotinine (KNHANES) were quantified using standardized laboratory assays. Multivariable linear regression and restricted cubic spline (RCS) models were employed to assess the relationship between cotinine levels and age at menopause, menarche, and RLS, adjusting for demographic, socioeconomic, and metabolic covariates. Subgroup analyses were conducted to explore effect modification. Results Higher cotinine levels were significantly associated with earlier menopause (NHANES β = −0.23; KNHANES β = −0.10) and shorter RLS (NHANES β = −0.22; KNHANES β = −0.08). RCS models confirmed linear dose–response associations in both cohorts, with threshold effects observed at higher exposure levels (NHANES ln-cotinine > − 3.47: β = −0.303, 95% CI: −0.386 to − 0.220, P < 0.001). Subgroup analyses indicated stronger associations among younger women, non-diabetic individuals, and lower-income groups, with pronounced differences across racial and educational strata. Conclusions Cotinine, as an objective biomarker of tobacco exposure, was robustly associated with shortened reproductive lifespan across two national cohorts. The associations were linear, with stronger reproductive toxicity at higher exposure levels, particularly among U.S. women. These findings highlight the reproductive risks of smoking and underscore the importance of biomarker-based assessments in reproductive aging research.