The investigators will categorise each of the adverse event outcomes as occurring within 3, 6, 9, 12, 24 and 48 months of first prescription. The investigators will do this because methods for conducting survival analysis using instrumental variables are not well developed. The investigators will use Stata 13.1 SE to generate all results. The instrumental variable analysis will be conducted using the ivreg2 command and psmatch2 will be used to construct the propensity score. All standard errors will be estimated using cluster robust standard errors which accounts for clustering of patients within practices.