Adverse drug events are the single leading threat to patient safety. Human factors engineering has been repeatedly proposed, but largely untested, as the key to improving patient safety. The value of this approach was investigated in the context of a commercially available patient-controlled analgesia device that has been linked with several alleged patient injuries and deaths. Several reports have stated that errors in programming drug concentration were made during these adverse drug events. A simulation of the commercially available interface was compared experimentally with a simulated prototype of a new interface designed according to a human factors process. Professional nurses, averaging over 5 years of clinical experience with the commercially available interface and only minimal experience with the new interface, programmed both interfaces. The new interface eliminated drug concentration errors, whereas the simulated commercially available interface did not. Also, the new interface led to significantly fewer total errors and faster performance. These findings may have broad implications for the design, regulation, and procurement of biomedical devices, products, or systems that improve patient safety in clinical settings.