Background: Medication-related adverse events (MRE) in anaesthesia care are frequent and require a deeper understanding if we are to prevent medication harm. Methods: We searched for reported MRE from the Spanish Anaesthesia Incident Reporting System (SENSAR) database over a 10-yr period. SENSAR is a cross-national, multicentre system focused on perioperative and critical care. A descriptive analysis of independent variables, phase of medication process, type of MRE, and medication group involved, and their relationships with morbidity was conducted. Results: A total of 1970 MRE were identified from 7072 reported incidents. Patient harm was reported in 31% of the MRE. The administration phase was more frequent (42%) and showed the highest harm rate (44%) compared with other medication process phases. The most frequent types of MRE were wrong treatment regimen and wrong medication (55% of cases). The medication groups most commonly reported were those that alter haemostasis (18%), vasoconstrictor agents (13%), and opioids (10%). Vasoconstrictor agents, benzodiazepines, and neuromuscular blocking agents were the medication groups involved in patient harm four-fold more, and opioids three-fold more, than medications that alter haemostasis. The 1970 incidents were investigated and led to implementation of 4223 local corrective patient safety and quality improvement measures. Conclusions: Patient harm in the perioperative setting from medications remains a major issue for patients, hospital leaders, and clinicians. We found patterns and specific causes that can be mitigated through proven systems solutions, and should be taken into consideration in designing sustainable solutions for safe perioperative care. Clinical trial registration: NCT03615898.