BACKGROUND: Uncertainty and risk are central issues in relation to health and health care services. Healthy individuals do not necessarily fall ill, despite the presence of risk factors. It has been documented that doctors, health service administrators and patients are more inclined to choose interventions against risk factors when information about the effects is presented in terms of relative risk reductions rather than absolute risk reductions.

OBJECTIVES: The objective of the study was to gain better insight into how GPs perceive risk of disease, and how this perception is influenced by the way the risk is presented, e.g. whether changes in risk are presented in absolute or relative terms.

METHODS: Questionnaires with clinical episodes were sent to 1500 Danish GPs. The GPs were randomized into four groups of 375, who all received the same case story with information about risk reduction achieved through medical treatment phrased in terms of either relative risk reduction, absolute risk reduction, number needed to treat or all of the aforementioned terms of risk reduction. The GPs were asked whether they would recommend medical treatment as primary prevention, knowing the case story and expected risk reduction.

RESULTS: The GPs' attitude towards recommending medical treatment was dependent on the phrasing of risk reductions. Seventy-two per cent of doctors who received all information on risk reductions would definitely or probably recommend medication, while 91% would recommend medication if information only about relative risk reduction was given, and 63% would recommend medication if information was given in terms of absolute risk reduction or number needed to treat.

CONCLUSION: In order to advise patients in a rational way, in addition to knowledge of the patients' preferences, doctors need to take into account all available measures of risk reductions.