;Situational' risk is an important determinant in the adoption of new pharmaceuticals. Not specifically the scientific risk of using the drug, but rather the risk the doctor perceives through his previous experience, both with similar treatments and with factors associated with the innovation such as its sponsor and mode of action. With this information the doctor determines to what degree further validation is required in order to allay any fears. The extent and complexity of this additional reassurance will thus determine how long is required before adoption and to what extent additional sources of information should be consulted.