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OBJECTIVE--To determine whether the reporting of study results by using reductions in relative or absolute risk and the number needed to treat affects the views of physicians about the effectiveness of drugs to lower lipid concentrations and decisions about treatment. DESIGN--Random allocation of two questionnaires presenting the results of three end points of the Helsinki heart study as results from separate trials by using reduction in either relative or absolute risk. In both questionnaires one end point was also presented by showing person years of treatment needed to prevent one myocardial infarction. The effectiveness of lipid lowering drugs was assessed for all end points on an 11 point scale. For each study result the likelihood to treat hypercholesterolaemia of 7.5 mmol/l in a healthy man had to be indicated on a seven point scale. SUBJECTS--Random sample of 802 internists and general practitioners representative of providers of primary care in Switzerland. RESULTS--The response rate was 69.6% (558). For the prevention of fatal and non-fatal myocardial infarction the mean ratings of effectiveness of lipid lowering drugs were 0.45 (95% confidence interval 0.21 to 0.69) and 1.39 (1.09 to 1.68) scale points lower when the reduction of absolute risk or number needed to treat were reported instead of the relative risk reduction (both P < 0.001). Physicians receiving trial results for identical end points in form of absolute reduction of risk or number needed to treat were less inclined to treat hypercholesterolaemia (both P < 0.001). CONCLUSIONS--Physicians' views of the effectiveness of lipid lowering drugs and the decision to prescribe such drugs is affected by the predominant use of reduction of relative risk in trial reports and advertisements.