This study analyzes the influence of medical prescriptions' writing on the occurrence of medication errors in the medical wards of five Brazilian hospitals. This descriptive study used data obtained from a multicenter study conducted in 2005. The population was composed of 1,425 medication errors and the sample included 92 routes through which medication was wrongly administered. The pharmacological classes most frequently involved in errors were cardiovascular agents (31.5%), medication that acts on the nervous system (23.9%), and on the digestive system and metabolism (13.0%). In relation to the prescription items that may have contributed to such errors, we verified that 91.3% of prescriptions contained acronyms and abbreviations; patient information was missing in 22.8%, and 4.3% did not include the date and were effaced. Medication wrong-route administrations are common in Brazilian hospitals and around the world. It is well established that these situations may result in severe adverse events for patients, including death.