The cohort we will use in the present analysis has been drawn from patients across 225 practices in England contributing data to GPRD, covering approximately 5% of the UK population (Figure 1). The study period is 1st January 2001 to 25th March 2010 (the date of the last GPRD data submission). To allow time for medical history and risk factors to be measured and recorded we require that patients enter the cohort after having completed at least 1 year of registration with their current practice during which the practice provides data that meet research quality criteria. Patients are followed up from the date they become eligible to be included in the cohort until the earliest date among a) the date of an initial presentation with one of the endpoints of interest, b) the date of leaving the practice or c) the date of last data submission from their practice.