Increased use of testosterone supplementation among older men in recent years has caused concern about the safety of exogenous testosterone administration. New formulations and delivery mechanisms, increased direct-to-consumer marketing, and wider recognition of natural declines in testosterone levels in older age and chronic disease have all contributed to wider use of testosterone therapy, without strong evidence of benefit or assurance of safety in older men who are already at risk for morbidity and mortality. Recent randomized trials have suggested an increased risk of cardiovascular events, but the body of literature is still without conclusive evidence. Additional analyses of large populations are needed to better describe how testosterone therapy is used in real-world practice and assess the possibility that use of testosterone may increase risk of serious adverse events. We propose a large retrospective cohort study using healthcare and laboratory testing data from a large population of older men in the United States with Medicare or commercial insurance. We will also create a second cohort using clinical data from the United Kingdom's General Practice Research Database. We will study patterns of initiation relative to testosterone testing, and how prescribing has changed over time. We will use propensity score techniques to compare rates of cardiovascular events in testosterone users to those of non-users with comparable testosterone levels. We will examine the risk of bone fractures and prostate cancer as secondary outcomes. These effects will also be investigated in vulnerable subgroups (e.g. older age, chronic disease, varying levels of testosterone deficiency). We will study transient risk of acute events using a self-controlled case-crossover analysis. And finally, different testosterone delivery mechanisms (e.g. patch, gel, injections) will be compared to determine if there are differential risks among the different formulations. The results of this stuy will contribute important evidence to the growing, but yet inconclusive, body of literature surrounding the safety of exogenous testosterone use among older men. PUBLIC HEALTH RELEVANCE: Testosterone supplementation is increasingly used in older men, although the benefits and safety have not been well-established. This study will use insurance billing information and laboratory test results from the United States and medical records from the United Kingdom to determine if taking testosterone supplementation increases older men's risk of serious events such as heart attack, stroke, other cardiovascular diseases, bone fractures, and prostate cancer. This study will use modern statistical methods and novel study designs to provide accurate measures of the risk of these events in men supplementing testosterone.