This thesis set out to address a clinical puzzle: after being offered statins, why do many people end up not taking them? This question is relevant to two different enterprises, one aiming to improve public health through individually-targeted preventive interventions, the other aiming to help patients make evidence-based decisions about such interventions. To answer it, I used elements of a grounded theory approach to analyse data generated by interviewing people who had been offered statins. In the resulting account of the way people come to take statins, I situate the cognitive work of decision-making within a web of social practices. Papers presenting many of my findings are incorporated into the thesis. The first draws on data from couple interviews, identifying ways in which these offer additional analytic purchase compared to individual interviewing. In the next two papers, examining the cognitive aspects of deciding about statins, I explore the way ‘need’ is constituted in relation to medication taken while feeling well. Participants reify test results, using them to account for medication decisions, but they do not use risk information in this way. This finding constitutes a challenge both to clinicians’ assumptions about communicating risk and to theoretical debates framing decision-making in terms of risk and uncertainty. The fourth paper focuses on the complex calibrations through which people negotiate tensions between conflicting norms concerning medication, negotiations complicated in the case of statins by the perception that they are a ‘lazy option’ chosen instead of a healthy lifestyle. This calibration work is required in order to construct a presentable account of ‘doing the right thing’; constructing this account is equated in the fifth and final paper with deciding what to do, a process that is inextricably entangled with other shared everyday practices. I conclude with two discussion chapters. The first contributes to debates about information, knowledge and expertise. The second relates these debates to the everyday clinical problem with which the thesis began; I explore the implications of my findings for the practice of talking with patients about statins, situating this exploration within the project of reshaping the collection of practices that constitute evidence-based medicine.