Smoking abstinence In the CPRD smoking status is indicated by whether the patient is a current, former or never smoker. As GPs are paid to record smoking status smoking behaviour is robustly recorded in the CPRD. Marston et al. (2014) found that 84% of patients had smoking status recorded within a year of registering at a practice, and that smoking prevalence rates by age were similar in CPRD and the Health Survey of England. Booth et al. (2013) found that the difference in prevalence of smoking estimate between the CPRD and the Health Survey for England was less than 1%, and the mean difference was 0.1% (95% CI: -1.5% to 1.7%). Using unpublished data from CPRD sampled as part of the research reported in Thomas et al. (2013) the investigators found that 74% of patients prescribed smoking cessation medication had a subsequent record indicating smoking status. Of these 66% were indicated as current smokers and 33% as ex-smokers. The investigators will initially define a patient as relapsed if they have any record indicating that the patient is a current smoker after their first prescription of a smoking cessation therapy. The investigators will not be able to determine the smoking status of patients who do not return to the GP. Therefore, the investigators will perform sensitivity analyses to examine whether the assumptions made about the smoking status of individuals who are not observed affect the results. For example, the investigators will conduct a sensitivity analysis to see if the results are altered by assuming that patients with missing data have relapsed, or by assuming that patients with missing outcomes have achieved abstinence.