How do doctors prescribe in older adults with multiple pathologies, and how do they deal with the problem of polypharmacy? This thesis sets out to answer this question by targeting the prescribing practices of physicians based on the articulation of three field surveys: in general practice, in geriatric consultations on hospital structures, and in a nursing home. The research is based on several types of sources : observations over several weeks of medical consultations and paramedical practices around drugs and prescriptions, around thirty interviews with doctors and paramedical professionals, and written sources (specialized journals, press reviews, documents and public reports on the topic, “old age and drugs”).This thesis has two ambitions : on the one hand, it is to develop a social science analysis of the problem of polyprescription, until now seen mainly through a medical and health prism; On the other hand, and more broadly, it is a question of developing a sociology of medical practices for the elderly.The monitoring of the problematization work of key players on this issue allows us first of all to shed light on the methods of emergence and transformation of the problem of polyprescription. The thesis shows, through the passage of the problem from a specialized sphere to the state sphere, the constitution of a discourse centered on the number of drugs which takes little account of the practices of doctors. She mainly tries to analyze the decisions of physicians to prescribe and deprescribe drugs, by highlighting the articulation of three dimensions : the medical profession and its regulations, the production and circulation of knowledge, and the institutions and of the organization of care. In town offices, if general practitioners use intuitive logic of global understanding of elderly patients to cope with the complexity of their cases, different ways of prescribing and managing prescriptions are structured according to both situations of individual interaction and the relationship they have to their profession, to other professionals and to medicines. The prevalence of uncertainties about medical and pharmacological knowledge in the practice of geriatricians working in hospitals has resulted in an explosion of positions concerning the usefulness and use of drugs. In this sector, prescription intervention often remains on the fringes of priorities, and a need for professional coordination to meet the challenges posed by the problem of polyprescribing. In nursing homes, the doctor’s prescribing practice is structured by an organization of care where nurses have knowledge of the overall condition of patients and their relationship to medication. This knowledge is thus the source of logic of cooperation between these actors who do the work of the doctor possible but contribute to maintaining a pharmaceuticalization problem. The thesis thus makes it possible to realize that the difficulties of prescribing and deprescribing as well as responding to the problem of polyprescription in older adults derive above all from the complexity of clinical cases and the specific nature of the situations to which each sector of care studied faces.