Medication prescribed within the hospital system determines the subsequent prescribing of drugs by general practitioners. This may explain why pharmaceutical companies sell certain drugs at very low prices to health establishments – it could be in the hope of increasing their retail market. To date, no empirical research has been carried out into this phenomenon. The purpose of this article is to quantify the impact of the choice of drugs (in nine competitive pharmacological classes) by hospitals (CHU) on the surrounding retail market in 2008. Two sources will be used : the DREES compilation on drugs used in health establishments and data extracted from the National Health Insurance Cross-Schemes Information System (SNIIRAM). In order to measure the simultaneous dynamics at play between these two markets, instrumental variables were used. Findings confirm that the purchase of drugs by hospitals has an impact on the quantities later prescribed for outpatients, and this impact can vary depending on the class of drug. This would suggest that regulation of drug expenditure should be tackled globally and not by separating the hospital and retail sectors.