Exposures First time users of the smoking cessation therapies (varenicline or nicotine replacement therapy) will be defined as people who received at least one prescription of the product after the 1st of September 2006 but with no use of a related product during the 12 months before the index date (the first date on which a prescription was issued). Langley et al. (2010) found the smoking cessation prescription data in the THIN database, which is closely related to the CPRD, to be highly comparable to national dispensing data. The analysis will be limited to the first treatment episode. This will mimic an intention to treat analysis in a RCT. The prescriptions will be defined by the therapy file in the CPRD, which contains a list of all prescriptions issued to patients at the practices. Each therapy record records the date a prescription was issued, the quantity of drug prescribed and the dosage.