In addition to their impact on human health, adverse drug reactions (ADRs) also have significant impact on healthcare costs. These costs are essentially hospital costs, in particular arising from an increase in length of stay caused by an ADR. Although it has been estimated that the occurrence of an ADR during hospitalisation or leading to hospitalisation is responsible for a cost of approximately EURO2800, several studies have also pointed out that the structure of ADR cost is heterogeneous, a factor which must be taken into account when developing preventive strategies. ADR cost evaluation remains difficult from a methodological point of view given that most studies have only evaluated the direct cost. Because of the substantial annual estimated cost of ADRs in industrialised countries, it is necessary to implement preventive programmes, with different strategies consisting of: educational programmes; identifying risk groups; implementing good drug practice; and clinical and laboratory monitoring for ADRs. Promoting pharmacoeconomic studies and co-operation between clinicians, medical pharmacologists and pharmacists remains the key factor for preventing ADRs and decreasing their costs.