Early age at smoking initiation is associated with elevated cardiovascular disease and mortality risk in a nationwide population-based cohort.

Smoking is a major cause of cardiovascular diseases, including myocardial infarction (MI) and stroke. However, the role of age at smoking initiation in cardiovascular risk has not been fully established. We selected 9,295,979 Korean adults without preexisting cardiovascular diseases who underwent health screening in 2009. The association of age at initiation with incident MI, stroke, MI or stroke, and all-cause mortality was analyzed using Cox proportional hazard models stratified or adjusted for total pack-years (PY). Among the 3,724,368 smokers, 876,740 smokers (23.5%) started before age 20, and 75,299 smokers (2.0%) had age at initiation < 15. Over a nine-year follow-up, ≥ 20 PY smokers with age at initiation < 20 had the highest risks of MI (adjusted hazard ratio (HR), 2.43; 95% confidence interval (95% CI), 2.35-2.51), stroke (HR 1.78, 95% CI 1.73-1.83), MI or stroke (HR 2.00, 95% CI 1.96-2.04), and death (HR 1.82, 95% CI 1.79-1.85) relative to nonsmokers, significantly higher than in ≥ 20 PY smokers with initiation age ≥ 20. Earlier age at initiation was associated with increasing PY-adjusted risks of each outcome (p for trend < 0.001 each). Age at initiation also had significant interactions with total PY for each outcome (p < 0.001 each), with PY-dependent risk increases accentuated by earlier initiation. Early age at smoking initiation is associated with increased risks of MI and stroke. Focused smoking prevention measures may be warranted for both adolescents and young adults.