Abstract Background and AimsType 2 diabetes mellitus (T2DM) is a chronic progressive condition characterised by hyperglycaemia due to insulin deficiency with or without insulin resistance. The prevalence of diabetes is increasing rapidly worldwide and it has a significant burden on health care resources with estimated costs of up to 10% of health expenditure in the UK. With an ageing population, people are now living longer with diabetes which consequently leads to increased multimorbidity and polypharmacy. Some previous studies have not assessed the effect of demographic or geographic factors (such as age, gender, UK nation and social deprivation) on the incidence and prevalence of T2DM in the UK over the past decade. In addition, detailed reports on the patterns of comorbidities in T2DM patients are sparse. Patients with T2DM are at a two-fold higher risk for cardiovascular (CV) disease. Some earlier studies assessing the CV risk associated with available therapies have been inconclusive in determining the preferred regimens. This thesis aimed to: i) assess the incidence and prevalence of T2DM in the UK; ii) investigate and compare mortality risk between T2DM patients and patients without diabetes and explore if it explains the observed prevalence rates; iii) examine the patterns of comorbidities in T2DM patients and matched comparators without diabetes; and iv) assess the comparative CV risk associated with second-line diabetes therapies. MethodsAll the studies in this thesis used data from the UK Clinical Practice Research Datalink. Access to linked national hospitalisation, deprivation and mortality data was obtained for the individual studies. Annual overall and gender-specific incidence and prevalence of T2DM were calculated for the study period 2004-2014. Rates were standardised by age bands, gender, neighbourhood social deprivation, and UK nation and expressed per 10,000 person-years (PYRs) with 95% confidence intervals (95% CI). For the mortality analysis, T2DM patients (cases) were matched to patients without diabetes (controls) on age, gender and general practice. Annual mortality rates for the matched T2DM and patients without diabetes were calculated and compared. Cox regression analysis was used to examine the effect of important covariates on the risk for all-cause mortality in the matched cohort and calculate hazard ratios (HR) and 95% CI. The multimorbidity profile in T2DM cases and matched controls, registered in English general practices, were also examined. Annual prevalence rates of 18 physical and mental health comorbidities were determined between 2004 and 2014 using linked primary care and hospitalisation records. For the CV risk analysis, patients prescribed a second-line medication after greater than or equal to90days of metformin monotherapy between 1998 and 2011 were identified. Using a retrospective cohort study design, inverse probability of treatment-weighted time-varying Cox regression models were used to estimate HRs and 95% CI for developing a major CV event (myocardial infarction, stroke, acute coronary syndrome, unstable angina, coronary revascularisation, or CV death) associated with second-line therapies after adjusting for clinically important CV risk factors.ResultsThe prevalence of T2DM nearly doubled from 320.62 (95% CI: 318.83; 322.41) in 2004 to 526.36 per 10,000 PYR (95% CI: 523.81; 528.91) in 2014, whereas the incidence was relatively stable with overall rate of 43.07 per 10,000 PYR (95% CI: 40.06; 46.09). Gender-specific incidence and prevalence rates were markedly higher in men than women. Between 2004 and 2014, the prevalence increased from 380.31 (95% CI: 377.48; 383.13) to 625.45 (95% CI: 621.37; 629.52) in men and from 268.56 (95% CI: 266.22; 270.90) to 437.28 (95% CI: 433.94; 440.62) in women. Overall, older individuals, men, and residents in the most deprived locations were more likely to have T2DM. Wales and Northern Ireland had higher prevalence rates than the other UK nations. In the all-cause mortality analysis, 20,312 (11.5%) patients with T2DM died, as compared with 79,951 (9.1%) controls. The adjusted survival model showed that patients with T2DM were at significantly greater risk for mortality in comparison with patients without diabetes (HR: 1.26, 95% CI: 1.20; 1.32). Mortality rates decreased over time in both cases and controls. The multimorbidity study showed that comorbidities were more prevalent in patients diagnosed with T2DM in comparison with patients without diabetes. The number of patients with two comorbidities increased between 2004 and 2014. The prevalence of cardiovascular disease (CVD) in T2DM patients was double that of matched control patients. DPP-4 inhibitors, thiazolidinediones and sulphonylureas add-on therapies to metformin were the most commonly-prescribed second-line therapies. The time-varying survival models showed that DPP-4 inhibitors (HR: 0.78, 95% CI: 0.55; 1.11) and thiazolidinediones (HR: 0.68, 95% CI: 0.54; 0.85) add-on therapies were associated with lower risk for major CVD compared to sulphonylurea add-on therapy when all added to initial metformin. Conclusions The prevalence of T2DM is increasing rapidly in the UK. Patients with T2DM are at significantly greater risk for mortality than patients without diabetes. However, with the declining mortality rates over the past decade, patients are now living longer to develop comorbidities. CVD was the most prevalent comorbidity in T2DM cases in comparison with people without diabetes. This is important as CVD is the main cause of mortality in patients with T2DM. Thiazolidinedione combination with metformin was associated with significantly lower CV risk in comparison with sulphonylurea add-on therapies to metformin. Lower, but non-statistically significant, risks were also found with DPP-4 inhibitors add-on therapies. These real-world findings add to the existing knowledge on the epidemiology of T2DM, provide novel insight on the patterns of multimorbidity in these patients and clinically relevant evidence on the CV risk associated with commonly-prescribed second-line regimens. Future larger studies are needed to confirm the observed CV benefits associated with antidiabetic therapies.