Since the 1980s, many measures have been implemented with the aim of controlling national health insurance drug expenditure. Although supply side policies are still widely implemented, the 1990s saw a move toward the concept of "medical practice cost containment" which aims to decrease practice variations by improving clinical processes and prescribing. However. several policies were unsuccessful and there is a need to develop a more coherent and effective policy agenda. This thesis aims to study prescribing variations in France and to explain them. It focuses on the primary care sector, where the greatest share or variation occurs, and on prescribing of proton pump inhibitors, the latest generation or gastric drugs. First, quantitative analysis or individual prescribing data is used to study variations and their determinants. Second, focus groups with general practitioners are conducted to better understand the routes or action or these determinants on the prescribing-decision process. Findings show that most variation is not clue to doctors' habits or preferences but to doctors adapting their choice to the particular case of the patient. The nature and severity or the clinical situation and doctors' perceptions or the drug's effectiveness are important in the decision making process. However. the mode or communication and social and interpersonal factors are also significant. Findings confirm the relevance of current policies aiming to decrease the influence of pharmaceutical companies and to develop doctors' lire-long learning. They also reveal a need for clinical guidelines adapted to the reality or practice and for training doctors to communication and to share decision-making with patients.