Introduction:
Most people who smoke initiate smoking in adolescence. Risk factors for smoking are changing over time as demographics shift, and technologies such as social media create new avenues for the tobacco industry to recruit smokers. We assessed risk factors associated with smoking uptake and regular smoking among a representative cohort of UK adolescents.

Methods:
Data come from 8944 children followed prospectively as part of the UK Millennium Cohort Study. Smoking uptake was assessed as adolescents who had never smoked tobacco at the age of 14 years, but reported smoking ≥1 cigarette per week by the age of 17 years (regular smoking). We used logistic regression to assess associations between smoking uptake and selected sociodemographic factors including household income, caregiver smoking, peer smoking, and social media use. Weighted percentages and Office for National Statistics Data were used to estimate numbers of regular smokers and new smokers in the UK.

Results:
Among the whole sample, 10.6% of adolescents were regular smokers at the age of 17 years. Of these, 52% initiated smoking between the ages of 14 and 17 years. Uptake was more common if caregivers smoked (13.6% vs 5.0%, p<0.001) or friends smoked (12.6% vs 4.3%, p<0.001), and among those reporting >5 hours/ day of social media use (9.8% vs 4.1%, p=0.006). Applying these percentages to population data, an estimated 160000 adolescents in the UK were regular smokers by the age of 17 years, of whom more than 100000 initiated smoking between the ages of 14 and 17 years.

Conclusions:
This analysis of smoking uptake and regular smoking highlight that smoking behavior remains highly transmissible within families and peer groups, reinforcing inequalities. Social media are highlighted as a potential vector.