A Twin Registry Study of the Relationship Between Posttraumatic Stress Disorder and Nicotine Dependence in Men

CONTEXT: Recent studies indicate a strong association between posttraumatic stress disorder (PTSD) and nicotine dependence (ND). However, the explanation for the association remains unclear. OBJECTIVE: To test competing explanations for the association between PTSD and ND. DESIGN, SETTING, AND PARTICIPANTS: Analysis of data on 6744 members of the Vietnam Era Twin Registry, a national registry of all male-male twin pairs who served in the military during the Vietnam era interviewed in 1991-1992. MAIN OUTCOME MEASURES: Risk of PTSD and ND using the Diagnostic Interview Schedule for the DSM-III-R. RESULTS: The prevalence of ND was elevated among trauma-exposed individuals (52.0%) and those with PTSD (71.7%) compared with unexposed individuals (40.5%). This association was significant for ND and for trauma without PTSD (odds ratio, 1.31; 95% confidence interval CI, 1.18-1.45) and for PTSD (odds ratio, 2.34; 95% CI, 1.92-2.84) and was not entirely explained by shared risk factors. Shared genetic effects explained 63% of the PTSD-ND association; the remaining covariance was explained by individual-specific environmental effects. Using survival analysis with time-dependent covariates, ND was associated with a substantially increased risk of PTSD among trauma-exposed men (hazard ratio, 1.98; 95% CI, 1.61-2.42). Trauma (hazard ratio, 1.49; 95% CI, 1.35-1.64) and PTSD (hazard ratio, 1.36; 95% CI, 1.14-1.61) were less strongly but significantly associated with increased risk of ND onset after controlling for shared risk factors. CONCLUSIONS: Most of the PTSD-ND association is explained by shared genetic effects. However, there is a substantial, robust PTSD-ND association not explained by shared risk factors. Multiple explanations for the association were supported; however, the strongest association was consistent with preexisting ND increasing the risk of PTSD onset. These data suggest that male veterans with a history of ND may be at increased risk for PTSD. Further research on the biological mechanisms underlying PTSD-ND comorbidity is needed.