BACKGROUND: The expression of drug concentration as a ratio may cause dosing errors. OBJECTIVE: To examine the effect of ratio expressions on drug administration. DESIGN: Randomized, blinded, controlled study. SETTING: Simulation center in an urban hospital. PARTICIPANTS: 28 physicians. INTERVENTION: Participants managed a simulated pediatric acute anaphylaxis scenario by using epinephrine ampules labeled with mass concentration (1 mg in 1 mL) or a ratio (1 mL of a 1:1000 solution). MEASUREMENTS: The amount of epinephrine given and the time taken to administer it. RESULTS: Compared with providers using ampules with mass concentration labels, those using ratio labels gave more epinephrine (adjusted mean dose, 213 microg above target [95% CI, 76.4 to 350.1 microg]; P = 0.003), and took longer to do so (adjusted mean delay, 91 seconds, [CI, 61.0 to 122.1 seconds]; P < or = 0.0001). LIMITATIONS: Performance in simulated scenarios may not reflect clinical practice. In reality, ampule labels provide both expressions of concentration. CONCLUSION: The use of ratios to express drug concentration may be a source of drug administration error. Patient safety might be improved by expressing drug concentrations exclusively as mass concentration.