In order to explore whether the association between tobacco use and any SRB is confounded by shared familial liability, we conducted conditional logistic regressions among twin pairs discordant for exposure and outcome. The first analysis was conducted among 66 female twin pairs discordant for age of regular tobacco use initiation and for any SRB. The point estimates suggested that those twin sisters with early regular tobacco use onset had higher likelihood for any SRB (OR 1.72, 95%CI 0.27–10.8, p=.56) and so did those who started regular tobacco use between ages 14 and 22 (OR 1.40, 95%CI 0.58–3.40, p=.45), when compared to their twin sisters who did not start tobacco use at all by the age of 22. Another pair-wise analysis included 157 twin pairs discordant for daily smoking status at baseline and any SRB at follow-up. The point estimates suggested that daily smoking co-twins had increased likelihood for any SRB (OR 1.94, 95%CI 0.65–5.75, p=.23) when compared to their never smoking co-twins. However, these associations failed to reach statistical significance. The small number of cases precluded further pair-wise analyses by zygosity (not shown in tables).
