This report was aimed at characterizing medication errors and evaluating their consequences for the patients' conditions and for the nursing workload in the Intensive Care Units (ICU) and Semi-Intensive Care Units (SICU) of two hospitals in the city of São Paulo. The sample was 50 patients, and data was gathered in record logs. The severity of the conditions and the nursing workload were assessed before and after the occurrence. Out of a total of 52 medication errors, 12 (23.80%), were non-administration of dosage, 11 (21.15%) were wrong medication, and 9 (17.31%) excessive dosage. There were no changes in patient conditions (p=0.316), but the nursing workload increased (p=0.009). As for the medication group, i.e, potentially dangerous or non-dangerous, there were no statistically significant differences either in the severity of the patients (p=0.456) or in the nursing workload (p=0.264) after the occurrence.