Power calculations The following power calculations are based on effect sizes and confidence intervals observed in the investigators previous analyses, which had data on 110,000 individuals prescribed either varenicline or nicotine replacement therapy. Based on the rate of 18,000 new prescriptions per year observed in the CPRD from 2006 to 2011, with a further 4 years of follow-up the number of patients prescribed either varenicline or nicotine replacement therapy will have increased by 72,000. Therefore the total expected sample size for analysis will be around 180,000.