The aim of this study was to assess the impact of differences in drug label information on injectable drug selection errors. Differences in the display of drug strength information were assessed in a randomised controlled trial involving ward nurses, intensive care nurses, nurse anaesthetists, ward physicians, and anaesthetists. A set of 24 on-screen tasks were constructed. For each task, a label corresponding to an instruction consisting of two from three possible pieces of information (concentration, quantity, volume) had to be selected from a list of 10 items. The set was presented three times to participants using three different label formats. Format A provided two pieces of strength information different from those in the instruction. Format B and C provided all three pieces in a random and a fixed sequence, respectively. The frequency of errors was statistically higher with formats A and B than with format C, and greater in nurses than in anaesthetists. Regulatory bodies should therefore implement a standard requiring that the concentration (expressed in 'mg x ml(-1)'), the amount and the volume of drug be displayed on medication labels in fixed locations.