Medication errors are common in hospitals, but only about 1 in a 100 actually results in harm to the patient. Conversely, only about 30% of injuries due to drugs in hospitals are associated with a medication error, and are thus preventable. Nonetheless, drugs are used so frequently that the total number of preventable drug injuries that occur is substantial, and these injuries are costly. Changing the systems by which drugs are ordered and administered holds substantial potential for reducing the number of drug-related injuries. Computerised ordering systems, in which orders are written on-line by a physician and the physician can receive feedback on the suitability of the order during the process of making it, appear likely to have an especially large impact on reducing medication errors.